Use pcase in option cell substitution

Also call option filters for all variable values, including `nil`.
Use `case` instead of `cond`

Simplifies the expression a bit.
Use cl-lib now instead of cl

Works fine under Emacs 24.3, but needs an additional compatibility
library on earlier releases.  Thankfully, package.el handles this for us.
Improve feature lists

Mention most features that were added of late, and highlight the
advantages of Flycheck more clearly.
Credit @magnars for his awesome s.el and dash.el

Many, many thanks, these libraries really make string and error
processing in Flycheck a breeze.
[Fix #110] Fix typo
Merge branch 'master' of http://github.com/lunaryorn/flycheck
Update README and doc files for elixir checker. Single semicolon in inlined comments.