Properly supporting sls deploy function command and zip option

[chongkong]

This PR fixes two issues I encountered while using individually: true option.

1. Should package only the given function when invoked by sls deploy function --function $FN_NAME

Suppose using individually: true. If you'd like to deploy a single function using sls deploy function command you'd expect the plugin to package only the module that contains the function specified. The current implementation, however, runs packaging for every module in serverless.yml definition, taking as much time as deploying all applications just to deploy a single function.

I simply added explicit targetFuncs property to filter target function if any specified in command line option.

2. moveModulesUp should be called regardless of zip option

When individually: true, each deployed function has its module as a root directory. This is achieved by calling moveModulesUp, but this method call is ignored when zip option is enabled. This certainly seems to be a wrong implementation (causing #203). If injectModule is ensured not to be called when zip: true, it is safe to call moveModulesUp whether zip is enabled or not.

The fix seems to work well on my case, but it lacks new unit tests (I couldn't figure out how to compose one.) If you let me known how to add a test, I'll update the PR as soon as I can.

[chongkong]

Oops lint error

[AndrewFarley]

LGTM, good catch

[dschep]

Nice! Thanks @chongkong!

I'd love it for this to be tested, but don't really know what the best way to test deployment 😢