Add cli options to output lookml

[remisalmon]

This should close #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

[remisalmon]

FileType objects dont work with context managers, this makes sure the file is closed even if they cannot be parsed.

[joshtemple]

Hey @remisalmon thank you for this PR and I'm sorry it's taken so long for me to review it. My only question is with the --write arg. Can you explain to me what you would use this for?

[joshtemple]

What's the use case for parsing from a file and writing back to the same file?

[remisalmon]

The lkml parser/dumper does a great job at cleaning up lkml files (separating measures from dimensions, correcting the indentation, etc.), the option to write it back to file would allow to use lkml as a auto-formatter in text editors, pre-commits, CI checks, etc. (like running black on .py files).

[remisalmon]

Hey @remisalmon thank you for this PR and I'm sorry it's taken so long for me to review it. My only question is with the --write arg. Can you explain to me what you would use this for?

Hi @joshtemple I have removed that option, happy to add it back if you feel it belongs here. I replied just above in #89 (comment).

I also added some cli tests to make sure the output (json or lookml) can be parsed back.