Improve logger to support HTML colors

[empijei]

Logger now outputs ANSI escape sequences to stdout.

It should be possible to set an io.Writer in the cli/lg package where the logger will write HTML formatted output. (it should do nothing if such writer is not set)

When now a line of output is foo \x1b[31mbar\x1b[0m
It should become foo <span style="color:#A00">bar</span>,

Some example here:
https://github.com/rburns/ansi-to-html/blob/master/src/ansi_to_html.js#L203

It would be better to wrap calls to log to directly output HTML-formatted outputs instead of converting it.

To achieve this a list as this one could be used, but for HTML instead of ANSI.

Implementing an "HTMLString" method like ShortString should work.

Please cover your code with tests.