Add derive flag

[vorner]

• There are currently two distinct *-derive features and it is uncomfortable to hunt for them.
• They exist mostly due to technical reasons, as they bring two different crates. The price of the other if one is already present is relatively low (the cost is by depending on syntax and quote and they share them).
• This is in the convention of having the derive feature flag as serde and other packages have.
• The original two are still preserved, so users still can enable only one of them.

Closes #684.

Checklist

• [x] I've added tests for all code changes and additions (where applicable) (not applicable)
• [ ] I've added a demonstration of the new feature to one or more examples Should I update the examples to use this feature, or leave them be with the current specs-derive?
• [ ] I've updated the book to reflect my changes
• [ ] Usage of new public items is shown in the API docs

I'm not sure this completely applies. I've updated the note in docs about features, but otherwise left the specs-derive as the main suggestion how to use it by users and expect this will be more like for people who go more by guess than by documentation. But if you prefer, I can update all the examples, etc, to use this new feature flag instead.

API changes

[azriel91]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push

[vorner]

Thank you :-)