plugin: add vsyntax and vfmt plugins

[nelsam]

This is the first pass at support for the V language. It contains some basic syntax highlighting, automatic 'v fmt' on save, and an update to the license header plugin to load for .v files as well as .go files.

I'll be adding some unit and maybe integration tests before merging this one, because I think vidar is starting to get mature enough to actually require thorough testing.

Type

• [ ] Bug Fix
• [x] New Feature
• [ ] Quality of Life Improvement

Tests

I have tested locally against:

• [ ] Windows
• [x] linux
• [ ] OS X
• [ ] BSD

I have included automated tests:

• [x] Unit tests
• [ ] Integration tests
• [ ] End to end tests