Support placement values and modifiers according to popper.js API (#259)

[tedbeer]

popper.js placement values are: auto|top|bottom|left|right and modifiers are -start|-end

Closes #259

[RobbieTheWagner]

@BrianSipple will merging this mess up your tippy refactor stuff and/or does the tippy refactor render this change obsolete? We should probably just pass the directions in, without checking against the regex and let the libraries that consume those directions spit out errors, right?

[BrianSipple]

@rwwagner90 There are a few pieces to that....

Tippy does support a placement option directly. However, we're currently parsing the placement setting out of the Step.attachTo option and merging it in to the tippy.popperOptions hash.

Ideally, we change attachTo to only be concerned with the element (see https://github.com/shipshapecode/shepherd/projects/2#card-13473896). But that's the kind of larger change we decided to hold off on until after the initial Tippy integration.

So with all that said... I think this change is good to go. (Thanks @tedbeer!)