Feature: compiler plugin

[xhd2015]

A general solution to #48

Provides the user a chance to inspect specific package's content, and modify them on the fly.

A possible implementation:

• When xgo compiles a package, it finds possible matching plugins,
• These plugins are go code which accepts a compile context as input, and return a Result describing what effects will be applied to the following compiling process, for instance, inserting a code snippet into beginning of a specific file

Plugin signature:

package plugin_demo

import (
     "github.com/xhd2015/xgo/plugin"
)

func Handle(ctx *plugin.CompileContext) *plugin.Result {
    return nil
}

Primary considerations:

• Performance
  • the plugin should be an opt in feature, if there isn't any, then performance should be as same as before
  • if there is some, ensure only invoke them when targets match
• Security
  • If user do not deliberately use it, then the compiler should bypass it
• Reusability
  • Plugin can be written in another separate module, then imported by different projects.