include lagoon alias commands to write alias file

[tobybellwood]

in addition to the drush la command that calls the API to output a list of aliases, we should include a command (or two) to generate an alias file - that can be output/piped to a file, or can be created in a given folder (in order to use with drush --alias-path)

This export would conform to the site alias specification, but would need to output the ssh port into the file (along with any other ssh options).

We would also need to consider whether there is an existing lagoon.site.yml and work out how to treat it.

[tobybellwood]

worth noting this issue with specifying alternate alias-paths - drush-ops/drush#5019