[ENH] Check literal blocks

[FHaase]

• closes Allow use literal blocks (::) #14
• write documentation

1. Splits blocks in *.rst files on occurence of .. highlight:: <language>.
2. If selected highlight-language is for python, search for literal blocks

Default language set by sphinx's highlight-language option can be set, as well as the languages that should be analysed.

[FHaase]

@kataev Do you mind having a deeper look into weather this actually works correct and I didn't miss any corner cases?

[kataev]

@FHaase we can run it again sqlalchemy repo :) and check output. I will try it a little later.