Enhance the indent function

[hairyhenderson]

From #162

It'd be useful to be able to call indent like so:

$ gomplate -i '{{ indent 4 "foo" }}'
    foo

This obviously assumes that the indent string is " " (a single space), and it needs to be possible to specify other indent strings. For this, a 3-arg form would be handy:

$ gomplate -i '{{ indent 2 "-" "foo" }}'
--foo

This implies that the default indent string would be " ".