Make dependency on aspell optional

joe is less than 2 MB in size and has no dependencies. Aspell
brings in 55MB in dependencies and more than 3000 files.

Change the port, so the dependency on aspell is optional (and
disabled by default) and allow installing aspell separately
for those who need it.

Reviewed by: bapt
Approved by: mentors (implicit)
Differential Revision: https://reviews.freebsd.org/D6732