Defining the SignifyDelete set of signs on the fly provides the ability
to specify the exact line deletion count for up to 99 lines, rather than
the current limit of 9 lines.
This also has the benefit of reducing the number of signs defined when
there aren't many hunks of deleted lines. On the flip side, if there
are other sign-placing plugins in use, then it also increases the chance
of nearing the ~120 sign definition limit if there are many hunks of
varying line deletions.
The [default] argument is used for setting the default highlighting for a group. If highlighting has already been specified for the group the command will be ignored. Also when there is an existing link.
