Allow for unopinionated conversion

[GuySartorelli]

Currently the conversion is opinionated - it enforces that the output must have some theme-specified foreground colour and some theme-specified background colour, even if the ANSI doesn't specify one.

Ideally for my use case it would only convert the specific ANSI formatting into HTML styles - and anywhere that ANSI hasn't specified formatting will be left to whatever the default CSS on the page dictates it should be.

This allows me to use OutputInterface to provide rich formatting when I need it (and have that converted to HTML with CSS), but the default output still respects the styling inherited from the page I'm outputting to.