Add -- for passing through flags to tsc

[varungandhi-src]

If you have an invocation like:

lsif-typescript index <args> -- <other-args>

We should pass through <other-args> to tsc and consume <args> within lsif-typescript.

[olafurpg]

I was going to implement this assuming it was an easy fix but commander merges arguments after -- with positional arguments. We can work around it by manually parsing out command.args (which includes the unparsed list of arguments) but decided to pause since it's unclear in what situations we need this functionality.