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>

test/fixers/test_gofumpt_fixer.vader

@@ -0,0 +1,27 @@
+Before:
+  call ale#assert#SetUpFixerTest('go', 'gofumpt')
+
+After:
+  call ale#assert#TearDownFixerTest()
+
+Execute(The gofumpt callback should return the correct default values):
+  AssertFixer {
+  \    'read_temporary_file': 1,
+  \    'command': ale#Escape('gofumpt') . ' -w -- %t'
+  \}
+
+Execute(The gofumpt callback should allow custom gofumpt executables):
+  let g:ale_go_gofumpt_executable = 'foo/bar'
+
+  AssertFixer {
+  \    'read_temporary_file': 1,
+  \    'command': ale#Escape('foo/bar') . ' -w -- %t'
+  \}
+
+Execute(The gofumpt callback should allow custom gofumpt options):
+  let g:ale_go_gofumpt_options = '--foobar'
+
+  AssertFixer {
+  \    'read_temporary_file': 1,
+  \    'command': ale#Escape('gofumpt') . ' --foobar -w -- %t'
+  \}