Add --end flag for add command

[michaelPotter]

Instead of specifying an event duration during creation, it can be
useful to give an event end time. This commit adds a --end flag that can
be used as an alternative to the --duration flag.

The --end flag takes a date string similar to the --when flag.

Given both a --end flag and a --duration flag, the duration value
will take precedence. Given neither flag, duration will be asked for
in interactive mode.

[jcrowgey]

I like this idea, can you rebase your changes upon current master?

[maxpereira]

this is a really excellent idea, any update on this?

[jcrowgey]

I think the idea here is sound, but we'd need to rebase upon current master in order to merge. Ideally, we would like to see some sort of unit tests added. @maxpereira if @michaelPotter isn't working on this anymore, is this something you could pick up to get it across the finish line?

[dbarnett]

I put together a version on top of current master in #708. Closing this stale PR in favor of that one.

Thanks!