Support reading C/C++ including path from EditorConfig files.

Add discussed in #220 and #215, 

> As long as there is no global architecture for configuration files for syntax checkers this one has to do the trick I guess. Comments and further suggestions are appreciated anyways.

This pull request will support [EditorConfig](http://editorconfig.org) as the global architecture of configuration for editors and IDEs, for the same purpose as #220.

EditorConfig has been used by [many projects](https://github.com/editorconfig/editorconfig/wiki/Projects-Using-EditorConfig), and is supported on [many editors/IDEs](http://editorconfig.org/#download), so I believe it is time to include this feature in syntastic.

This pull request read a property called `c_include_path` in the `.editorconfig` files, which [will be added in the official standard in the future](https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties). This feature also depends on the [EditorConfig Vim plugin](https://github.com/editorconfig/editorconfig-vim), and takes use of its [hook mechanism](https://github.com/editorconfig/editorconfig-vim/blob/682f9ff1ae0ebba1edfafb6d5d1a1fd190ddfcda/doc/editorconfig.txt#L143), which was first introduced in editorconfig/editorconfig#45. 

Wish this could be included. Thanks!