Let the build_arg_string function handle GMT configurations #2299
Description
Originally posted by @seisman in #2208 (comment)
When working on the Figure.timestamp function, I feel sometimes we may need to pass GMT's configurations directly to the GMT C API, i.e., passing a string like --FONT_LABEL=xxxx to the call_module function.
I propose to add a new parameter (e.g., confdict) to the build_arg_string function, which takes a dict and convert key-value pairs to the form of --key=value, e.g.,
build_arg_string(kwdict, confdict={"FONT_LOGO": font, "FORMAT_TIME_STAMP": timefmt}