Add --dry-run to eb per issue #654

Adds a --dry-run option to eb. When used, prints the list of
easyconfigs to be built and whether each one currently exists,
then exits. If --robot is used, also does this for each of their
dependencies.
Fix --dry-run option per comments in PR

* Split dry-run out into a separate function
* Change output syntax to be more easily greppable
* Convert camelCase variables to snake_case
Merge pull request #677 from ajdecon/dryrun

Add --dry-run to eb per issue #654