Add api_key_placeholder option

[SofiaSousa]

Related to #113

[dblock]

Looks great, merging.

Couple of things that I think would make sense as another PR if you're up for it?

1. I think when the value is set to nil, it should not appear in the form. Same for other values.
2. We need a reset! for options to clear/revert to default api_key_default_value and other values around each spec.

Btw, interested in co-maintaining this lib? It doesn't see a lot of action. If you are, drop me a note to dblock[at]dblock[dot]org with your rubygems alias and maybe you can make the next release with this feature?