allow to forward nim cmd line params to nimble: nimble build|install [nimble options] -- [nim options]

[timotheecour]

as discussed on IRC
https://irclogs.nim-lang.org/04-01-2019.html#22:42:20

currently impossible to pass un-supported options, eg : --debugger:native ; instead all nim options should be passable as-is (without further checks, nimble shouldn't care about verifying it) after --

IIRC parseopt supports passing remaining args

EDIT(2019-01-29):
I was going to open another issue to make nimble build --verbose (ditto w install) more informative (currently no way to tell what it's doing when building a binary, besides the vague Building nimterop/nimterop/toast using c backend which doesn't tell me anything about options being used (eg doesn't tell if -d:release is used ; turns out its used for install but not build) ; however I realized the best approach is just to fix this (#599), which enables more flexibilty; eg it would allow: nimble build -- --verbosity:2 --hints:off to address this particular point

[dom96]

Implemented in 5ec2ece

[timotheecour]

example (for reference):

nimble install -y -p:-u:release https://github.com/genotrance/nimgen

however, even with --verbose, nimble doesn't show the actual command used and instead shows the un-informative:

   Building nimgen/nimgen.exe using c backend

ideally it should simply show the command used, eg

nim c -o:nimgen/nimgen.exe -u:release nimgen/nimgen.nim

=> fixing it in #781