As an enhancement to #594, we can support nested quotes.

For example, when splitting on the "," split regex, with a Map option whose keys have embedded '=' characters, we want to be able to parse input like this:

<cmd> --option "'keyprefix=keypostfix'=value","'key2=key2'=value2"

Giving a map with key-value pairs like this:

keyprefix=keypostfix : value
key2=key2 : value2