Bugs: Detect list type switching and support child deltas in lists

• Handle custom colour attribute in a better way, assign to style attribute if sensible, thank you Alex. Accepted PR as is, only handles colour, will extend feature in v3.18.0.
• Fixed #108, links within headers not correctly rendering, headers now support child deltas.
• Fixed #109, not correctly closing paragraphs or detecting list type change.
• Removed the ParserSplitInterface interface, no use anymore.
• Changed which class implements ParserAttributeInterface interface.
• Removed redundant methods from parsers.
• Code formatting and comment corrections.

Rework of splitting retain newlines

• Reworked the parser, split deltas much sooner to try and simplify some of the later logic.
• Return deltas pretty much exactly as defined, most of the new lines intact, I'm thinking about
  adding a strict mode that will generate what I consider to be better HTML.
• Update to the README explaining my plans for v4.
• Reworked a couple of tests and removed one or two that I did not consider to be useful.
• Coding standard correction thanks to Lode Claassen

Support for `Color` delta type

• Added support for Color delta type, it creates a span with a style="color: #xxx" definition,
  thank you (https://github.com/on2)

Notes

The implementation works and does not break anything, however, I will extend the feature in the future.
It should really be handled in the same way unknown attributes are handled in the plain Insert delta.
The Insert delta only deals with a simple key and value, it needs to be extended to allow complex values,
that way an attribute like {"attributes":{"style":{"color": #e60000"}} could generate the same output and
allow much more flexibility. I'm happy to leave the Color delta in until the next major version as I
believe it is a feature other people may use.

Multiple newline hotfix

• New paragraph not correctly being created for final sub insert when insert split on multiple newlines.

New line only inserts, bug fix

• New line only inserts no longer ignored by the parser, updated a couple of tests because of the logic update.
• Updated README, added new credit.

Multiple attributes ignored by Markdown parser [Bugfix]

• Multiple attributes ignored by the Markdown and the GitHub Markdown parsers, added two tests to catch regressions.
• Minor changes to README and composer.json.

Markdown - GitHub flavour

• Initial support for GitHub flavoured Markdown, extends Markdown and adds support for strikethrough text.
• Added basic escaping for HTML and Markdown.
• Reorganised the tests ready for additional file support and features.

Refactoring, duplication in parsers

• Removed duplication in HTMl and Markdown parser, moved similar logic to the base parser class.
• Updated composer dependencies.
• Added additional Quill info to README.

Markdown parser list fixes

• The Markdown list parsing is now correctly using child deltas and matches the HTML parser.
• Added additional list tests, Markdown versions of the HTML list tests.

More list fixes

• List parsing code was ignoring previous items that were closed, incorrectly adding them as children.
• Single item lists not correctly ending.
• Lists where the last character or the entire list item was a non insert delta type rendered incorrectly.
• Deltas can now return their assigned attributes.
• Pass attributes into each HTML delta.
• A slew of tests to test lists.

I'm hoping this is it from now with regards lists.