Accept multiple paths for combine command

This pull request allows you to supply multiple paths to the `prmd combine` command.

While generating documentation I'm splitting up endpoints to be on their own page.
Being able to supply multiple files I can pull in the shared parameters (my way of solving https://github.com/interagent/prmd/issues/70).

Example:
```bash
$ prmd combine --meta ./meta.json ./schemata/shared-definitions.json ./schemata/activity.json 
```