Improve handling of color in prompts

[lzybkr]

This PR is one possible approach to fixing #986.

It's nice in that it doesn't require adding a new configuration option - I'm thinking this would be used very infrequently.

Here is an example of how you might configure PSReadLine and an example of it working:

$esc = [char]0x1b
$pc = [char]0xe0b0
$fg = "0"
$bg = "8;2;95;158;160"
$ebg = "1"
Set-PSReadLineOption -PromptText "$esc[4${bg};3${fg}mPS$esc[4${fg};3${bg}m$pc$esc[0m","$esc[4${ebg};3${fg}mPS$esc[4${fg};3${ebg}m$pc$esc[0m"

If we go with this approach, I'll also need to update the docs.

[lzybkr]

Note that if you try my sample, you will want an powerline font installed and set the output encoding to utf8.