Improper Handling of Nested Formatting

[Ste1io]

telebot.apihelper.ApiTelegramException: A request to the Telegram API was unsuccessful. Error code: 400. Description: Bad Request: can't parse entities: Can't find end of Bold entity at byte offset 3676

Originally posted by @sudoskys in #70 (comment)

Nested formatting is not being properly handled, when parsing the syntax tree specifically at:

telegramify-markdown/playground/markdownify_case.py

Lines 78 to 79 in 9a81090

	*bold _italic bold ~~italic bold strikethrough ||italic bold strikethrough spoiler||~~ __underline italic bold___ bold*
	__underline italic bold__

From the Telegram API docs:

In case of ambiguity between italic and underline entities __ is always greadily treated from left to right as beginning or end of an underline entity, so instead of ___italic underline___ use ___italic underline_**__, adding an empty bold entity as a separator.

[Ste1io]

@sudoskys,

Had a chance to look at this tonight, and made good progress. I will finish implementing remaining changes and open a PR tomorrow. As a bonus, strikethrough is treated correctly for both functions for their expected syntax input, and properly ignored if invalid (see screenshot below) - one of the last unchecked todo items.

Nested tokens required a more explicit regex matcher that excludes matches if preceded/followed by an additional concurrent token delimiter. Also, additional rendering has to be handled in both renderer classes to prevent it from propagating up the chain and being handled using the default markdown renderer.

I'll open a PR with the remaining nested bugfix changes fully implemented for you to look over when I'm back at pc tomorrow. See current (improved) nested handling with my partial fixes in screenshot below.

[sudoskys]

cc #55

[sudoskys]

This issue is marked as unresolvable. I will try to rework everything using the new method.
#81