Support crontab expressions

[kwando]

It would be awesome to be able to create a schedule with a crontab expression.

[kwando]

I will probably need this is one of my projects and will probably take a stab at implementing it.

[avit]

I've added a couple examples of parser classes for Hash and Yaml, maybe it's a helpful pattern for this if you want to tackle it.

[avit]

Related: #232