Implement gofumpt Fixer (#3968)

* Implement gofumpt Fixer

Add an implementation with test and documentation for the gofumpt go
code formatter, a stricter formatter than your standard "go fmt".

Signed-off-by: David Houston <houstdav000@gmail.com>

* Add gofumpt to ale.txt TOC

Forgot to add gofumpt to the ALE vim help Table of Contents, so do so.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Fix Test Setup Method Capitalization

I had put "Setup" instead of "SetUp" for "ale#assert#SetUpFixerTests".
Fix such.

Signed-off-by: David Houston <houstdav000@gmail.com>

* Fix typos

Add a missing space, remove an extra bracket by actually running tests
locally first. Would've been smart to do that from the beginning...

Signed-off-by: David Houston <houstdav000@gmail.com>

doc/ale-supported-languages-and-tools.txt

@@ -182,6 +182,7 @@ Notes:
   * `go mod`!!
   * `go vet`!!
   * `gofmt`
+  * `gofumpt`
   * `goimports`
   * `golangci-lint`!!
   * `golangserver`