Add body getter and support multipart/form-data enctype

[elquimista]

This PR supports req.body getter. E.g. I was using jquery-ujs for an anchor tag with data-method="delete". Since jquery-ujs is originally designed for rails, it used form body to send hidden input field value with name of _method. And this obviously doesn't get passed query getter.

[coveralls]

Coverage remained the same at 100.0% when pulling 731ac2b on clthck:master into b1ca349 on koa-modules:master.

[coveralls]

Coverage remained the same at 100.0% when pulling daec4cd on clthck:master into b1ca349 on koa-modules:master.

[coveralls]

Coverage remained the same at 100.0% when pulling daec4cd on clthck:master into b1ca349 on koa-modules:master.

[fundon]

Is it better by an option?

[elquimista]

I think this is improved version for sure. Original one can only handle situations where _method param comes in query string, not request body or multipart/formdata

[fundon]

I think we can use an option for enabling that.

[elquimista]

@fundon what's your plan for this PR?

[fundon]

@clthck About an option?

[elquimista]

I don't think providing this as an option is not appropriate. It's not something user wants to opt in/out - it should be handled by default. There's no difference in overhead or whatever if we make this feature optable.