Allow group specification in ArgInstall options

Allow group specification in ArgInstall options

The option that add a line to the existing multi-line options
(e.g. -installTheme which adds to -themeNames) now can specify the
line should be added within a group, by prefixing the line with
"# GROUPNAME\n" (where \ and n separate characters). If a group of
that name already exists, the line to add  is insert immediately after
the group header line. If not, the group is created, by adding both the
group header line and the line to add, as well as an "# end" line to
terminate the group.