Add Tag support to sysl modules

[cuminandpaprika]

Current Sysl Version

$ sysl info
Build:
  Version      : 0.83.0
  Git Commit   : f5b3d9a
  Date         : 2020-05-08T05:26:49Z
  Go Version   : go1.13.10 linux/amd64
  OS           : darwin/amd64
Runtime:
  GOOS/GOARCH  : darwin/amd64
  Go Version   : go1.13.10

Use-cases

As a sysl user, I would like to be able to import various versions of sysl files in a similar way to go module versioning.

Proposed solution

import //github.com/anz-bank/sysl/demo/simple/hello.sysl@v1.0.0
App[~ignore]:
    ...

The above snippet would import hello.sysl at v1.0.0

import //github.com/anz-bank/sysl/demo/simple/hello.sysl@my-branch-name
App[~ignore]:
    ...

The above snippet would import hello.sysl at v1.0.0