Use helpers to get the desired colors

This allows someone who's adding support for a new colorscheme to use
logic when choosing which colors they want (like what's currently
happening with solarized), but avoids the problem of keeping all the
available colorschemes in memory.

This also reduces the need for guards, we only need one per variable,
because if the user hasn't specified which colors they want (or set the
boolean indicating they want the legacy colors) then we're free to
choose what we think is best for them based on what we know about their
colorscheme, possibly falling back to defaults if we don't know about
their colorscheme.
Add documentation about adding colorscheme support
Remove legacy colors/add g:niji_colorus/update doc

- New configuration option: `g:niji_colorus` that allows you to override
  the current color set regardless if there's a dark/light background.
- Remove the `g:niji_use_legacy_colours` variable
- Update the docs
- Legacy colors now show up as an example in the docs
Move color/char setup/guards into their helpers
Allow for more flexible color set formats

- A list of pairs (this is what niji#highlight() actually needs)
  e.g. [['blue', 'darkslateblue'], ['darkmagenta', 'darkviolet']]
- A map containing light/dark keys that each have lists of pairs
  e.g. { 'light': [['blue', 'darkslateblue']], 'dark': [['darkmagenta',
'darkviolet']] }
- A list of strings (for when you don't need different colors for the
  console and the gui)
  e.g. ['red', 'blue', 'green']
- A map containing a list of strings, one each for light and dark
  backgrounds
  e.g. { 'light': ['blue', 'red'], 'dark': ['yellow',
    'green'] }

Note: I've smoketested this a little, and it seem to work just fine :).
This also means we can use these simpler formats in
`niji#get_<colorscheme>_colours()` functions, because their return value
will be checked and expanded as needed.
Show supported color set formats in the docs

Note: I've tested the examples and they work (but they're all red :P)
Remove light/dark color set variables

These are covered by using `g:niji_colours` with a map that has
light/dark keys in it. See `:help niji-colour-set-format` for more info.
Add support for `g:niji_colorscheme_colors`

This has been tested and documented, and it takes precedence over
`g:niji_colours`.

*'niji_???_colours'* in the docs could probably have a better name
though.
Merge pull request #8 from joshuarh/helpers-and-different-color-set-formats

Add helpers, different color set formats