Allow using pointer receivers, and use struct pointers for input parameters

[misberner]

This PR changes two things:

• It adds a boolean ptr_receivers flag (default false), that, when set to true, causes generated Marshal[Easy]JSON methods to be generated with a pointer receiver instead of a value receiver.
• It causes internal, generated marshaling functions to expect input structs to be passed as pointers instead of values.

This PR unconditionally reverts the effect of #15 on the generated internal functions, and allows conditionally reverting the effect of #15 on the generated external (exported) methods.

The motivation is to allow easyjson to work with the new Google protobuf API (see fixed issue).

Fixes #348 .

[misberner]

Could any of the maintainers please take a look at this?

[misberner]

@bulletmys @GoWebProd would it be possible to take a look at this PR?

[misberner]

@gobwas any chance you could look at this?