:Color for modify current color, and keep format

Add a new command `:Color`
This command will query current color and keep the format(`#fff` or `rgb()`).
If no color on current cursor position.
Then will use default format, which is HEX format.

It's a pity that applescript don't support alpha channel.
So what I can do is keep the value if I can get one.

2 new command also introduce:
* `:ColorRGBCSS`, will use `rgb(0, 100, 255)`
* `:ColorRGB100`, will use `rgb(0%, 10%, 100%)`

Other small fix included in this pull request are:
* Default color set to white
* Improve 255 <-> 65535 transform
* Code tuning
* Other minor bug fix

Might add HSL format support.
Studying how to transform it.