Request: New bind property for route parameters

[goenning]

Issue Description

It is common for POST/UPDATE requests to have the resource identified sent on the URL while the content is sent on the body. It'd be useful to have the DefaultBinder to bind route parameters into the Model. Example below.

Checklist

• Dependencies installed
• No typos
• Searched existing issues and docs

Expected behaviour

type newCommentInput struct {
	Number  int    `param:"number"`
	Content string `json:"content"`
}


input := new(newCommentInput)
c.Bind(input);

This would bind both Number and Content.

Actual behaviour

It is necessary to use both c.Bind and c.Param('number') to fill this model.

Steps to reproduce

n/a

Working code to debug

An example of how I'm currently doing it https://github.com/getfider/fider/blob/557d3c515b8b5ac55ae1c0f7161d2d11b448fef7/app/handlers/idea.go#L131-L152

Just as information, this is common in other frameworks. Following example from C#.

[HttpPut("{id}")]
public IActionResult Update([FromRoute] string id, [FromBody] TodoItem item);

Version/commit

v3.2.1

Thanks for creating Echo :)

If approved, I could give it a try and work on it.

[vishr]

@goenning Should be ok