Fixes #1092, Adds a format option to the {{isotime}} variable.

Now using the golang magic date: Mon Jan 2 15:04:05 -0700 MST 2006
One can format the time like:

    {{isotime "2006-01-02"}} == "YYYY-MM-DD"
    {{isotime "060102-15"}}  == "YYMMDD-HH" (24-hour clock)
    {{isotime "060102-3"}}   == "YYMMDD-H"  (12-hour clock)

Using `{{isotime}}` as a standalone variable doesn't change. It still returns **RFC3339** formatted time.