allow idl to pin meta version to master

[cl1337]

version control in idl seems limited to a pinned sha, ideally we want more feature rich version control to be supported by idl, like ranged version, minimal version, or master version. Repo use idl should do some extra work to update versions

to begin with, one feature we want most is pin master to avoid some frequent updated idl schema repo with churning meta

[cl1337]

@kriskowal let me know what do you think, I can help make this change. ^