[TW-1801] Allow recurrence to accept specific days of the week

[taskwarrior]

Alan Young on 2016-05-09T23:54:26Z says:

Allow something like 'recur:Mon,Tues,Thur' to mean each Monday, Tuesday and Thursday.

[taskwarrior]

Migrated metadata:

Created: 2016-05-09T23:54:26Z
Modified: 2017-01-16T21:09:47Z

[djmitche]

@carmiac has proposed a major revision to recurrence support in https://github.com/carmiac/taskchampion/blob/798cec2b40f13c420ef6358e4d99b251e58249de/docs/src/iterative-tasks.md. If you're following this issue, please have a look and comment in GothenburgBitFactory/taskchampion#729.