detect Vimball (.vba, .vmb) files as Vim script

[smola]

Fixes #501

Description

• added .vba and .vmb extensions to Vim script
• added .vba disambiguation
• added Vimball sample with .vba extension (Vim license) from https://github.com/nelstrom/vim-textobj-rubyblock/blob/master/textobj-rubyblock.vba
• added Vimball sample with .vmb extension (WTFPL) from https://github.com/CDSoft/todo/blob/master/todo.vmb
• added Visual Basic sample with .vba extension (MIT license) from https://github.com/Akaizoku/concatenate/blob/master/procedures.vba

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on github.com
    • Search results for each extension:
      • https://github.com/search?q=extension%3Avba+usevimball+NOT+nothack&type=Code
      • https://github.com/search?q=extension%3Avmb+usevimball+NOT+nothack&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/nelstrom/vim-textobj-rubyblock/blob/master/textobj-rubyblock.vba
    • Sample license(s): Vim License
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[pchaigno]

Thanks a lot @smola for fixing this!

I left a few comments below.

[pchaigno]

Thanks for the fix!