plainbox:color: add Colorizer.is_enabled

This patch adds a new property to the Colorizer class. Using the
Colorizer.is_enabled property applications can determine if actual
rendering is using colors and, for exampe, change a verbose piece of
text to a shorter, color-coded symbol.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:color: add Colorizer.custom()

This patch adds a new utility method to the Colorizer class. The
Colorizer.custom() method can be used to combine a piece of text with
any ansi escape code in a reliable way, even if color support is
disabled.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/colorizer/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"