Support for multile filetypes in clang_completer

clang_completer would check if the raw value of '&ft' was one of
supported filetypes for the completer.

Vim allows for multiple filetypes with a '.' separator. A file with
ft=qt.cpp, for example, would not be supported by clang_completer even
though it was a cpp file.

This patch changes that behaviour.
Check for a native completer with multiple filetypes

GetFiletypeCompleter would always return a omnicompleter for the first
filetype in case there was no native completer, and the lookup would
stop.

This changes that behaviour to get all possible completers and tries to
find a native one among them. If no native completer is found, it
returns the omnicompleter for the first filetypes, as it used to.
Select correct triggers for multiple filetypes

The base class completer Inner chose the first filetype available and
would use the triggers for it.

The triggers are now chosen considering the first for the current buffer
that is supported by the current completer. If there is no intersection,
it fallsback to considering the first filetype for the buffer.
Merge pull request #172 from zehrizzatti/multiple_filetypes

Fix for multiple filetypes set