feat, refactor: make use of new parser for nicer cmp preview docs

[smjonas]

Now the parser introduced in #20 is utilized to provide more info to cmp items. This info is used to improve the cmp documentation / preview window (currently defaults to a hardcoded implementation showing the description and snippet contents).

Snippets with option 'r' or 'e' are parsed but not passed on to cmp at the moment.

Here is a screenshot for a snippet with the following definition:

snippet tabt "Some description"
The snippet contents
endsnippet

The documentation is provided by a function that takes the snippet info table as a parameter and returns a string. We could let users overwrite this so they can display whatever they want, e.g. also show the options associated with a snippet.

I have tested this and it should work (also the tests still pass). But please test this PR first before merging :) Suggestions for improvement / changes are welcome.

[smjonas]

Force pushed to remove cur_info.ft = in line 30

[quangnguyen30192]

Thanks, tested it and displayed nicely