Make git blame's date output format configurable, like git log

Make git blame's date output format configurable, like git log

Add the following:

 - git config value blame.date that expects one of the git log date
   formats (e.g. relative,local,default,iso,...);

 - git blame command line option --date expects one of the git
   log date formats;

 - documentation in blame-options.txt;

 - git blame uses the appropriate date.c functions and enums to
   make sense of the date format and provide appropriate data;

git blame continues to line up the output columns by padding the date
column up to the max width of the chosen date format.

The date format for git blame without both blame.date and --date continues
to be ISO for backwards compatibility.

git annotate ignores the date format specifiers and continues to uses the
ISO format, as before.

Signed-off-by: Eugene Letuchy <eugene@facebook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>