Proposal: Add a build API for building Go program in go/build.

Context: `go build` and `go test` use an internal build API to build a Go program
(http://golang.org/src/cmd/go/build.go?#L359). It'd be nice that the API is available to
third party software as well. Google Group discussion is here
(https://groups.google.com/forum/#!topic/golang-nuts/0FBoVbLyfkw).

Use Case: `gotask` (https://github.com/jingweno/gotask) adopts similar approach as `go
test` to write and run task functions. Similar to how `go test` compiles and run test
functions, `gotask` compiles task functions on the fly and run them. The difference is
`go test` uses an internal build API (http://golang.org/src/cmd/go/build.go?#L359) while
`gotask` uses the `go build` command. I'm wondering if it makes sense to add a new build
API for building Go program in go/build so that 3rd party software can make use of. 

Thanks,
Owen