Add jv_dumpf() and jv_show()

jv_dumpf() takes a FILE *.

jv_show() is intended for use in debuggers, so it dumps the jv to stderr
and it does not jv_free() the jv, so it's safe to
"call jv_show(some_jv, -1)" in a debugger.  If flags == -1 then the jv
will be shown pretty-printed and in color.