Highlighting customisation tweaks.

Highlight groups for signs:

- renamed highlight groups with `GitGutter` prefix, and following the
  naming style of the builtin groups `DiffAdd`, `DiffChange`, etc.

- using `highlight default link` to set default colours for signs.
  e.g. the plugin defines `GitGutterAddDefault` as green, and default
  (if `g:gitgutter_highlights`) links `GitGutterAdd` to it (which the
  user can customise).

Highlight groups for lines:

- added gitgutter-specific highlight groups for line highlighting
  (`GitGutterAddLine`, etc) so that this highlighting can be customised
  independent of `DiffAdd` et al (which these default link to).

- line highlighting toggling now only changes the linehl attribute.

  The `linehl` attribute is toggled in `s:update_line_highlights`,
  which separates concerns more clearly.  I'm thinking ahead to the
  possibility of the sign text being configurable, and thinking it will
  be DRYer to keep these things separate.  Maybe it's just aesthetic
  though.

Bonus: resolved "eugh" ;)
Define sign column highlights. Add colours and signs section.
Extract method for clarity.
Construct variable on one line for clarity.
Easier customisation.