Eliminate 4-bit colors, add RGB conversion tools, HTML parse mode

[Volte6]

Description

This eliminates support for 16-color (4-bit) colors, an unused feature that increases complexity.
It also adds a new parsing mode to convert tags into HTML output (<span>)

Changes

• Renaming color256: in ansi-alias file to simply colors
  • color256 remains supported.
• Eliminated code related to 4-bit colors
  • Updated unit tests
• Added rgb.go - provides RGB(int) which accepts a color code and returns an rgb struct/object
  • unit tests
• Added ansitags.HTML as a parse mode for output.

Notes

Since an XML-like tag structure is used for tags, you could really cheat this with string (or regex) replacements, but this maintains the ability to process a stream of data, such as that piped in from another program.
Whether that's useful or not remains to be seen.

Example

Here is a side-by-side of a web-render of the html output vs. an in-terminal output using ansi escape codes: