Add fmt flag to specify formatter (#117)

- Support formatting output with gofmt(default) or goimports via flag.
- Introduce moq.Config struct to configure moq.Mocker instance. This
  breaks backward compatibility but facilitates it in the future if and
  when any features are added.
- Use golden file tests for validating formatters. Use 
  github.com/pmezard/go-difflib to print the diff between expected and
  actual.

README.md

@@ -24,7 +24,9 @@ $ go get github.com/matryer/moq
 ### Usage
 
 ```
-moq [flags] destination interface [interface2 [interface3 [...]]]
+moq [flags] source-dir interface [interface2 [interface3 [...]]]
+  -fmt string
+	go pretty-printer: gofmt (default) or goimports
   -out string
     	output file (default stdout)
   -pkg string
@@ -33,6 +35,9 @@ Specifying an alias for the mock is also supported with the format 'interface:al
 Ex: moq -pkg different . MyInterface:MyMock
 ```
 
+**NOTE:** `source-dir` is the directory where the source code (definition) of the target interface is located. 
+It needs to be a path to a directory and not the import statement for a Go package.
+
 In a command line:
 
 ```