Improve highlighting for Elixir.

[pkaminski]

Specifically:

• Add missing keywords 'require', 'import', and 'with'.
• Correctly (not) highlight the '::' operator, instead of treating as symbol.
• Allow for compound names (for modules, but affects other definitions too).

Fixes #1824.

[marcoscaceres]

Few test failed on TravisCI. Could you please check?

[pkaminski]

Looks like the shell detection test is failing, perhaps because I added the with keyword and the test fixture has the line Your branch is up-to-date with 'origin/master'.. Is there an easy way to exclude a keyword from contributing to language detection? I see some language definitions with relevance: 0 on rules but not on keywords.

[pkaminski]

Never mind, it was indeed the with keyword and I found the docs on how to zero out its relevance. All tests passing now.

[marcoscaceres]

Oh, great work @pkaminski! Thanks for figuring it out!