support forwarding nim flags, eg:`choosenim --skipUserCfg 1.0.8` 
`choosenim 1.0.8` fails because of things in my user cfg which may not be supported in the version I'm building.

option 1:
user would add logic in their user cfg, eg:
`when (NimMajor, NimMinor, NimPatch) < (1, 3, 5):`
this isn't very convenient and makes it awkward for 1-off builds.

option 2:
`XDG_CONFIG_HOME= choosenim --skipUserCfg 1.0.8`
this works in this case but is blunt and can have other side effects

option 3 (this issue):
allow nim flags in choosenim, that get forwarded, eg:
`choosenim --skipUserCfg 1.0.8` 
or (like in `inim --flags:'--lib:lib '`) via:
`choosenim --flags:'--skipUserCfg -d:foo' 1.0.8` 

