Support disabling of hashid signing feature

[olliebennett]

This PR fixes #30 by implementing a configuration option, sign_hashids, which controls how the underlying hashid is generated. The choice is:

sign_hashids = true (default)

• hashid is generated by encoding a "signed" array; encode_id([42, id]).
• this logic was implemented in 941bc3e.
• this signed array is validated during decoding, to identify that the correct ID has been obtained (i.e. when decoding, we confirm that the decoded value is of the form [42, x] and return the x.
• the benefit is that when decoding a value which could be either a raw ID or a hashid, it's possible to determine which it is; a raw ID that may (by chance) be a valid hashid (for a different ID) will not be mistakenly decoded into that ID.
• the disadvantage is a slightly longer hashid being generated.

sign_hashids = false

• hashid is generated simply from the id itself; encode_id(id).
• this was the behaviour for hashid-rails < 1.0.0; in order to maintain backwards compatibility for existing hashids, this must be set.

[jcypret]

Nice work. The changes look good.

[jcypret]

@olliebennett I'm going to add an additional note in the readme and get a new version of the gem pushed. Thanks for the contribution!

[jcypret]

Added a note to the readme with upgrade instructions and warning others about the changing ID behavior.

[olliebennett]

Thanks a lot for the quick turnaround, Justin! :) Glad to be back on the bleeding edge of the gem.