Extend sequence API

[wekyle]

The sequence API is kinda weird, in that you usually end up passing an array of words broken by spaces, for example ['npm', 'install'].

I've yet to see a use case that breaks that pattern, actually.

We could save users some keystrokes (and headaches) by allowing them to pass in a string (for example, npm install, which will be broken down into the command and array of arguments to be passed into exec behind the scenes.

[wekyle]

Backwards compatibility would be nice as well

[wekyle]

Let's add support for promise-returning functions as well.

[wekyle]

As of d6772b2, we prefer sequences to be arrays of strings, not arrays of arrays as before. It helps that child_process.exec, which we adopted in #1, accepts commands with arguments as a single string.

This is simpler, easier to type, and easier to read when editing a target!

I've added in legacy support for arrays for backwards compatibility- this will be removed...someday....

Arbitrary functions are forthcoming.