Add self-contained gometalinter build tooling.

tools/vendor/github.com/mvdan/interfacer/README.md

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+# interfacer
+
+[![GoDoc](https://godoc.org/github.com/mvdan/interfacer?status.svg)](https://godoc.org/github.com/mvdan/interfacer)
+[![Build Status](https://travis-ci.org/mvdan/interfacer.svg?branch=master)](https://travis-ci.org/mvdan/interfacer)
+
+A linter that suggests interface types. In other words, it warns about
+the usage of types that are more specific than necessary.
+
+	go get -u github.com/mvdan/interfacer/cmd/interfacer
+
+### Usage
+
+```go
+func ProcessInput(f *os.File) error {
+        b, err := ioutil.ReadAll(f)
+        if err != nil {
+                return err
+        }
+        return processBytes(b)
+}
+```
+
+```sh
+$ interfacer $(go list ./... | grep -v /vendor/)
+foo.go:10:19: f can be io.Reader
+```
+
+### Basic idea
+
+This tool inspects the parameters of your functions to see if they fit
+an interface type that is less specific than the current type.
+
+The example above illustrates this point. Overly specific interfaces
+also trigger a warning - if `f` were an `io.ReadCloser`, the same
+message would appear.
+
+It suggests interface types defined both in the func's package and the
+package's imports (two levels; direct imports and their direct imports).
+
+### False positives
+
+To avoid false positives, it never does any suggestions on functions
+that may be implementing an interface method or a named function type.
+
+It also skips parameters passed by value (excluding pointers and
+interfaces) on unexported functions, since that would introduce extra
+allocations where they are usually not worth the tradeoff.
+
+### Suppressing warnings
+
+If a suggestion is technically correct but doesn't make sense, you can
+still suppress the warning by mentioning the type in the function name:
+
+```go
+func ProcessInputFile(f *os.File) error {
+	// use as an io.Reader
+}
+```