color: support a different color mode when the pager is active
MSYS on Windows has a terminal that supports the "win32" color mode
(which "auto" properly detects for us). However, a popularily configured
pager in that environment (GNU less) only supports the "ansi" color
mode.
This patch teaches color about a new config option: pagermode. It
behaves like "mode" but is only consulted when the pager is active for
the current command. MSYS users can now set "pagermode = ansi" and get a
colorful experience that just works. Previously, MSYS users would have
to live without color when using GNU less as the pager, would have to
manually configure the pager to attend every command, or would have
gibberish if "ansi" was used without the pager.