Support a list or organizations in shallow cmd

[carlosms]

Fix #1.

Done on top of #32, only the last commit is relevant. I did not change the argument name, but maybe we want it to be --orgs/GHSYNC_ORGS

Did some quick tests and it seems to work without any further problem.

[smacker]

yes. let's replace arguments to orgS. Similar to the change that was made in gitcollector:

To collect repositories from several organizations GITHUB_ORGANIZATIONS=src-d,bblfsh , the env var has changed, note the plural in github_organizationS

[carlosms]

Done in 354b174