Allow pass-through flags for spago docs

[elliotdavies]

purs docs accepts a --format flag that can be used to generate Markdown documentation and Vim/Emacs tags as well as HTML (the default). Currently spago docs doesn't accept any arguments, so you always get HTML. Could we allow e.g. spago docs -- --format ctags in the same way that spago build accepts pass-through arguments?

If we think it's a good idea, I'm sure I could PR this!

[justinwoo]

i've needed this sometimes as well, and using the same mechanism is probably easiest

[f-f]

@elliotdavies sounds good!

However, I'd prefer to not pass arbitrary arguments to purs docs, but have --format as a Spago flag. This is because we'll need to do other stuff than just calling purs when having html output (see #89)
If @justinwoo needs other flags other than format, we can add them as Spago flags too

[elliotdavies]

@f-f OK great - makes sense to me. I'll get on it!