Allow to specify a GitHub pull request to merge into the downloaded templates

Should hopefully help with template-library-core#38
get-template-library: add --help, clarify usage
get-template-library: fix --pull-request for repository with a master branch
when using tag rather than HEAD
get-template-library: reduce default verbosity
get-template-library: detect/signal a wrong tag and exit with a non zero status
Merge pull request #44 from jouvin/get-template-library

Allow to specify a GitHub pull request to merge into the downloaded templates