ensure git is properly configured

[ghistes]

Hi,

when running dstask for the very first time it should ensure that git is usable.

This means:

• it should bail out (with an appropriate error message) should there be no git installed
• prompt the user for email and name in case git is installed but not configured and use this to configure git so that it is usable

[Coleman McFarland]

I would not endorse actually writing to their git configs. However an error message that includes a prompt "is git installed?" could be fine during the first-run initialization, but maybe not on every invocation.

So I like the first suggestion, but not the second so much