Add #rrggbb string parsing

[kilpkonn]

I find it more convenient to keep colors in #rrggbb format so added support for that

Thanks for the grate lib btw!

[BurntSushi]

I am open to doing this, but I want to make sure this has no breaking changes. I don't think there are any, but could you please think through that and provide an argument as to why?

Also, this will need:

• Added documentation, since this adds public API surface. For example, here: https://docs.rs/termcolor/1.1.2/termcolor/enum.Color.html
• Tests. This should be pretty thoroughly tested.

[kilpkonn]

I added some tests and limited the string length to be 7 so #0011FF1 would fail rather than act like #0011FF.

My reasoning for why it's not braking change:

1. It only has effect where the command previously "failed" eg. is supposed to be ANSI code but actually is not.
2. It requires string to have fixed length of 7 and start with # so I does not conflict with other formats.
3. It doesn't panic if something is wrong but just return Err as it should.

For documentation, did you mean a line there or would you like some short example usage?

[BurntSushi]

For documentation, did you mean a line there or would you like some short example usage?

It should document the format, at minimum. e.g., "A # followed by 6 hexadecimal digits of any case, with each octet representing red, green and blue, respectively."