Support for 'default' value in segment background

[gagarski]

When a terminal theme is bright (with black font color), the context segment content is invisible (because it is black on black). Here I set the default foreground color to white for this segment.

[vadviktor]

I have checked this fix in KDE5 Konsole and @gagarski has a point, on white background themes the context segment is busted, all black. The change does not seem to affect further use on other colourschemes, all looks good.

@dawikur @caiogondim any objections?

[dawikur]

It makes a point, but maybe the better way would be not to set color neither on BG nor FG, instead of forcing it on both?

[vadviktor]

Well, the other contexts also have their "default" colours, it doesn't feel like we are forcing it but setting a sensible default. Am I not right?
Also, these are just defaults and can be overridden.
As I see it with the white value, a newbie user comes with his white BG terminal and will NOT see the theme "buggy". :)

[gagarski]

@dawikur As far as I understand we cannot draw an arrow () with transparent FG and blue (default for path segment) BG using terminal escape sequences. That's why black BG is forced.

[gagarski]

@dawikur

As far as I understand we cannot draw an arrow () with transparent FG and blue (default for path segment) BG using terminal escape sequences.

Looks like we can and I now am playing around with this, so I'll push update when I test it.

[gagarski]

So, now we can set 'default' as background for any segment. Did it by default for custom and context segments. Rebased everything on top of master branch.

[gagarski]

Rebased everything on top of master

[gagarski]

@dawikur @caiogondim Is it ok or should I change/update something to get it merged?