Add a command to 'degap' workspaces. (-g)

This causes all numbered workspaces to become consecutive. Note that
this does not influence the order, only the numbering.

For example, if the workspaces are [1, 3, 5, 7], then 3 will become 2, 5
will become 3, and 7 will became 4, which results in the consecutive
workspace order [1, 2, 3, 4].
Change the format string syntax in a function.

move_window_here was using the old-style format strings (%). It's now
using the up-to-date format method.
Rework the usage documentation.
Version bump to 2.1.
Update PKGBUILD.