Support main package not in root

[roycephan]

What does this PR do?

• Add config main_package
• Add go mod

Description

With projects have main.go is not in root. Fresh needs to watch files in root & build main_package

Example config:

    root:                       .
    main_package:      .
    ......

[brhamidi]

What is the status of this MR ? this is exactly what i want!
The fresh project is still being maintained?

[roycephan]

@brhamidi I created this MR to contribute to this project in order to other gophers can use (if it's necessary). Accepting this MR is belong to this project's owner.

I only use my fork.