Separating lib from cli

[heynemann]

This PR corresponds to issue #654.

It extends Revive usage as a library to more than just cli usage. Clients can now use parts of the revive workflow (Lint, Format) and pass configuration to the flow themselves, instead of loading from a toml file.

After this PR users can use the library with:

        // removed error handling for clarity
	conf, _ := config.GetConfig("./defaults.toml")

	revive, _ := revivelib.New(
		conf,  // config
		true,  // set exit code
		2048,  // max open files
		[]string{"./tests"},  // exclude
		revivelib.NewExtraRule(&mockRule{}, lint.RuleConfig{}),  // extra rules
	)

	// Now they can lint
     	failuresChan, _ := revive.Lint("../testdata/if-return.go")

	// Get the []lint.failure list
        failureList := revive.GetLintFailures(failuresChan)

        // Or just get the formatted output and exit code
	failures, exitCode, _ := revive.Format("stylish", failuresChan)

[heynemann]

@chavacava can you review this PR? If you want I can change revivelib to just lib. I just felt like doing:

revive, err := revivelib.New(...)

Feels nicer than lib.New(...).

[chavacava]

Hi @heynemann, I did not forget your PR, I'll try to check it this we. Thanks!

[chavacava]

LGTM, I've left a couple of comments/questions but nothing relevant.
Let me know when you are ok to merge

[heynemann]

Sorry for the initial low quality of this PR. It feels much better now. If you feel like it is up to par, just merge it :D

[chavacava]

@heynemann thanks for the PR! Nice work.