feature request: Support build a project as monorepo-like

[miyukki]

In my project I needed to support a monorepo-like structure that there is go.mod in each folder.

For example it has below structure:

└── serverless-project
    ├── node_modules
    ├── functions
    │   ├── func1
    │   │   ├── go.mod
    │   │   ├── go.sum
    │   │   └── main.go
    │   └── func2
    │       ├── fuga
    │       │   └── fuga.go
    │       ├── go.mod
    │       ├── go.sum
    │       └── main.go
    └── serverless.yaml

So I added the monorepo flag into config and I improve this plugin that it specified function directory to cwd on build command when enable monorepo flag. Maybe the test code is easy to understand in detail.

[mthenw]

Hey! Thanks for the contribution. Could you also update the Configuration section in the readme?

[miyukki]

@mthenw Thanks! And I update README.md to describe it.

[nguyenvanduocit]

I didn't know about this PR until I created PR #16. :|