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>

autoload/ale/fix/registry.vim

@@ -246,6 +246,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['go'],
 \       'description': 'Fix Go files with go fmt.',
 \   },
+\   'gofumpt': {
+\       'function': 'ale#fixers#gofumpt#Fix',
+\       'suggested_filetypes': ['go'],
+\       'description': 'Fix Go files with gofumpt, a stricter go fmt.',
+\   },
 \   'goimports': {
 \       'function': 'ale#fixers#goimports#Fix',
 \       'suggested_filetypes': ['go'],

autoload/ale/fixers/gofumpt.vim

@@ -0,0 +1,17 @@
+" Author: David Houston <houstdav000>
+" Description: A stricter gofmt implementation.
+
+call ale#Set('go_gofumpt_executable', 'gofumpt')
+call ale#Set('go_gofumpt_options', '')
+
+function! ale#fixers#gofumpt#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'go_gofumpt_executable')
+    let l:options = ale#Var(a:buffer, 'go_gofumpt_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ale#Pad(l:options)
+    \       . ' -w -- %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction