Narrow allowed shortcut format

[hanslovsky]

The endpoint format is very well defined in README.md/the usage message.

The endpoint should have one of the following formats:

- groupId:artifactId
- groupId:artifactId:version
- groupId:artifactId:mainClass
- groupId:artifactId:version:mainClass
- groupId:artifactId:version:classifier:mainClass

AFAIK no such format restriction exists for shortcuts. To keep things sane, we should probably restrict the shortcut format to something like [A-Za-z0-9-_]+.