FR: Support for hexadecimal strings

[Jlll1]

Many applications require use of hexadecimal representation for colors (for example 0x0000aa).
Currently nvim-colorizer does not highlight these. This feature request is about adding support for hexadecimal color representation and highlighting them as well.
Example:

[Akianonymus]

This is just valid for #0000aa or #FF32A18B too ?

[joshpetit]

This would be helpful since in Flutter all the colors are specified in Hexadecimal, so I would +1 this as well!

[catgoose]

Should be fixed with #91