Add new '%EXT' placeholder for custom toolchains

[ashthespy]

Follow up from #89 and #90 that should provide a solution for #86 and #42

For changelog (Changed)

[(#97)](https://github.com/James-Yu/Atom-LaTeX/pull/97) Add new ``%EXT`` placeholder for custom toolchains

[James-Yu]

Will release soon.

[ashthespy]

Can you hold on for a few hours? Will add more small stuff :-)

[ashthespy]

Sorry, had a rather busy weekend - but should be all good now!
Would you like me to edit the pull requests and add a line with information for the Changelog.md file?

[James-Yu]

That would actually be awesome!

[ashthespy]

Done, but this might save you more time ;-)

### Changed
* [(#50)](https://github.com/James-Yu/Atom-LaTeX/pull/50) Preliminary `arara` log parsing support
* [(#97)](https://github.com/James-Yu/Atom-LaTeX/pull/97) Add new `%EXT` placeholder for custom toolchains
* [(#98)](https://github.com/James-Yu/Atom-LaTeX/pull/98) Add `.bib` to subfile-autocomplete + code clean up
* [(#100)](https://github.com/James-Yu/Atom-LaTeX/pull/100) Add error notification for invalid LaTeX root file
### Fixed
* [(#102)](https://github.com/James-Yu/Atom-LaTeX/pull/102) Fix root directory determination