please support "-tag" syntax to remove metadata values

[zacchiro]

[ context: this thread on the Ledger-cli maling list https://groups.google.com/forum/#!topic/ledger-cli/AfGKE2Svv0s ]

To remove a metadata tag "FOO", icsv2ledger supports the "-FOO" syntax, which is great.
However, to remove a metadata value "FOO:BAR", one currently has to write "-FOO:BAR" which can get quite tedious when "BAR" is a long string. It would be nice if icsv2ledger would support the "-FOO" syntax to remove the whole "FOO:BAR" metadata value.

With many thanks for maintaining icsv2ledger!