Include code from scripts inside Markdown documents

[achjaj]

I added support for including lines from Julia script. Following syntax is used: `js = "path to script" => [lines]`
For example:
`js = "curves.jl" => [1; 5:10;]`
will be replaced with line 1 and lines from 5 to 10 from sript curves.jl

[codecov-io]

Codecov Report

Merging #407 (42d928f) into master (c9fc740) will decrease coverage by 0.73%.
The diff coverage is 27.77%.

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
- Coverage   73.73%   73.00%   -0.74%     
==========================================
  Files          26       26              
  Lines        1348     1363      +15     
==========================================
+ Hits          994      995       +1     
- Misses        354      368      +14     

Flag	Coverage Δ
unittests	73.00% <27.77%> (-0.74%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/reader/markdown.jl	78.12% <27.77%> (-19.84%)	⬇️
src/rendering/texformats.jl	83.96% <0.00%> (-0.95%)	⬇️

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