Configure pflag map/valued flags with option to set `:` as a delimiter

[carlisia]

Currently, pflag requires the format of key1=val1,key2=val2 for map/valued flags.

We prefer to use the : delimiter (as in key1:val1;key2:val2 instead of =.

Would a PR to allow for delimiter customization be useful?

[fmenezes]

here at MongoDB we would benefit from this change as well