fix issue #22 cmd line arg to set compiler and get help; fix issue #16

[timotheecour]

./inim -h
Usage:
  main [optional-params]
inim interpreter
  Options(opt-arg sep :|=|spc):
  -h, --help                      write this help to stdout
  -n=, --nim=      string  "nim"  path to nim compiler
  -s=, --srcFile=  string  ""     nim script to run
  --showHeader     bool    true   show program info startup

--showHeader:false is useful to use inim as a cmd line tool:

echo 'import os; echo os.findExe("dmd")' | ./inim --showHeader:false
/Users/timothee/homebrew//bin/../Cellar/dmd/HEAD-81082d5/bin/dmd

--nim is useful to allow using a custom nim binary, eg nim_temp produced by ./koch temp

[AndreiRegiani]

@timotheecour Really nice contributions! Will be released as v0.3.0
Thanks for reporting the other issues as well 👍