Highlight watched Expressions that have changed.

[rcaputo]

This might address issue #24, too.

[cyrus-and]

This is probably good enough, the only problem is that format_value may return an ANSI string and nested styling does not work, in fact it is only highlighted the part of the value up to :, which is visually acceptable I guess but it might be source of bugs/surprises in the future. I'll think about it...

[rcaputo]

I suppose the matter of highlighting the value could be sidestepped entirely by adding a separate indicator. Perhaps a single character in front of the expression.

[cyrus-and]

True, but the highlighting was nice actually and coherent with the rest.

[cyrus-and]

This simple trick should do, it's not an attempt to strip all the ANSI codes, only those concerning the formatting.