Add support Phoenix Live View (Live EEx)

[vermaxik]

Since Phoenix Live View become more a more popular,
I would like to see highlight source code in GitLab/GitHub for *.leex extensions.

Basically it alias for *.eex, that a reason why it was added in existing lexers.

[pyrmont]

Thanks for the PR @vermaxik! :)

I should note:

• GitHub used to use Rouge for syntax highlighting but now uses its own code so unfortunately this won't help there. To have GitHub syntax highlight .leex files as Embedded Elixir, you'll need to submit a PR to GitHub's Linguist library (I'm not very familiar with this project but I think this is the file you need to edit).

• GitLab does use Rouge, though, and once they upgrade to the version of Rouge that incorporates this PR, it should start working there. We're on a two-week cadence for releases and the next version is due out this coming Tuesday.

[vermaxik]

Thank you @pyrmont