Implement custom snap version string generation

The snap version string generated by the `git` value of the `version`
key isn't mimicing the previous versioning style due to the prefixing
`v` in the git tag, this patch replace it with a custom implementation
based on the `version-script` key.

Note the `version` key is still required to exist due to schema
requirements, but has no effect when the `version-script` key is
defined.

Reference:

    Snapcraft version scripts - snapcraft - snapcraft.io
    https://forum.snapcraft.io/t/snapcraft-version-scripts/106Signed-off-by: N林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

snap/snapcraft.yaml

 name: glances
-version: git
+version: set-by-version-script
+version-script: git describe --always --dirty --tags --match 'v*' | sed s/^v//
 summary: Glances an Eye on your system. A top/htop alternative.
 description: |
   Glances is a cross-platform monitoring tool which aims to present