Add vimtex support

Indicates:
- whether the file is considered to be main or local
- whether the viewer is opened
- whether the compilation is running
- whether the compilation is continuous

Added:

* `vimtex` existence check
* variables documentation

TODO: readme and a screenshot

Update readme.md

Update doc

Update screenshot url

README.md

@@ -105,6 +105,9 @@ vim-airline integrates with a variety of plugins out of the box.  These extensio
 #### [xkb-switch][48]/[xkb-layout][49]
 ![image](https://cloud.githubusercontent.com/assets/5715281/22061422/347e7842-ddb8-11e6-8bdb-7abbd418653c.gif)
 
+#### [vimtex][53]
+![image](https://cloud.githubusercontent.com/assets/1798172/25799740/e77d5c2e-33ee-11e7-8660-d34ce4c5f13f.png)
+
 ## Extras
 
 vim-airline also supplies some supplementary stand-alone extensions.  In addition to the tabline extension mentioned earlier, there is also:
@@ -253,3 +256,4 @@ MIT License. Copyright (c) 2013-2016 Bailey Ling.
 [50]: https://github.com/jreybert/vimagit
 [51]: https://github.com/Shougo/denite.nvim
 [52]: https://github.com/Shougo/dein.vim
+[53]: https://github.com/lervag/vimtex