Extract Option classes from cli.toit into options_.toit (#99)

cli.toit had grown to ~1856 lines. Move all Option-related classes
(Option, OptionString, OptionEnum, OptionInt, OptionPatterns,
OptionPath, OptionInFile, OptionOutFile, OptionUuid, Flag, InFile,
OutFile, CompletionCandidate, CompletionContext) into a new
options_.toit file and re-export them from cli.toit.

Author: floitsch