Improve syntax.

- Update list of keywords.
- Highlight all prototypes.
- Comments: special highlighting for the following words: TODO, XXX,
  NOTE, FIXME, BUG and HACK.
- Comments: special highlighting for sequence of capitalized words,
  ending in a colon. e.g.:
    // Highlighted Comment Title: Non highlighted comment text.
- Strings: highlight interpolated elements.
- Numbers: better error highlighting for improper hex and octal numbers.
Improve indentation.

- Consider continued lines.
- Consider multi line string.
- Consider multiple opening/closing parens in previous line.
- Allow customization of some features.
Merge branch 'syntax'

Conflicts:
	syntax/io.vim
Merge branch 'indent'