Add cli options to output lookml

[remisalmon]

This should close https://github.com/joshtemple/lkml/issues/88.

> lkml --help
usage: lkml [-h] [-v] [--json | --lookml | -w] file

A blazing fast LookML parser, implemented in pure Python. When invoked from the command line, returns the parsed
output as a JSON string.

positional arguments:
  file           path to the LookML file to parse

options:
  -h, --help     show this help message and exit
  -v, --verbose  increase logging verbosity to debug
  --json         return a JSON string (default)
  --lookml       return a LookML string
  -w, --write    parse and write back to file