Support ddoc as description for helptexts

[gizmomogwai]

I was playing with rust/clap and one feature I really liked there was, that you can use doc-comments as an alternative to annotations to document flags/commands... Not sure that its possible to get to the doc-comments in dlang though (I found an old issue: https://github.com/dlang/dmd/pull/6872 where one of the usecases was especially commandline help generation).