Allow custom timestamp for `get_local_node_and_date()`

[engnadeau]

Expected Behavior

• Ability to set custom timestamps (e.g., %Y%m%d%H%M%S instead of %Y%m%d)
• Multiple dirty builds are sometimes generated in quick succession

Actual Behavior

• Hardcoded %Y%m%d timestamp: https://github.com/pypa/setuptools_scm/blob/46df188a54e3eb2b0dfb93dbae59fb23249fbcce/setuptools_scm/version.py#L140-L144

Specs

• Python 3.6.1
• setuptools-scm 1.15.6

[RonnyPfannschmidt]

this scheme is registered as node-and-date = setuptools_scm.version:get_local_node_and_date
for a higher resolution it should be relatively simple to regster something like a node-and-timestamp scheme

the quickest way to get it is a own setuptools_scm plugin,
the second quickest is to make a pr with it so i can merge/release
i wont to any feature-dev this year