Refactor and trap localStorage and JSON parsing errors

[tbcd]

The main reason for this PR is to trap exceptions being thrown by localStorage (of which there are many weird cases across different platforms and browsers), and JSON.parse().

Exceptions are logged to the console as warnings, by default, re-using disableWarnings to silence them. This means that disableWarnings is now needed as an option for save() and clear() as well.

Also includes some trivial refactoring in digestible small commits that steer toward the final commit.

[kilkelly]

Thanks @tbcd , great pull request 💯 Sorry for the delay in merging it!

[tbcd]

No problem at all. There was quite a bit more in this one than the others so I expected it to take more time, assuming you had actually wanted it.. as it did come out of the blue. :)

I didn't mark this PR as fixing #23 because that was requesting something a bit different, but these changes may be similar enough to close that issue anyway.