add: support for nickel format as Nickel fixer (#4665)

Nickel(https://nickel-lang.org/) is a configuration language, like
Jsonnet, Cue, Dhall.

`nickel`(https://github.com/tweag/nickel) is the main command to run,
export and also format Nickel code.

this commit adds `nickel format` as a Nickel fixer, together with some
tests and documentation.

test/fixers/test_nickel_format_fixer_callback.vader

@@ -0,0 +1,27 @@
+Before:
+  Save g:ale_nickel_nickel_format_executable
+  Save g:ale_nickel_nickel_format_options
+  Save &l:expandtab
+  Save &l:shiftwidth
+  Save &l:tabstop
+
+After:
+  Restore
+
+Execute(The nickel_format callback should return 'nickel format' as default command):
+  setlocal noexpandtab
+  Assert
+  \ ale#fixers#nickel_format#Fix(bufnr('')).command =~# '^' . ale#Escape('nickel') . ' format',
+  \ "Default command name is expected to be 'nickel format'"
+
+Execute(The nickel executable and options should be configurable):
+  let g:ale_nickel_nickel_format_executable = 'foobar'
+  let g:ale_nickel_nickel_format_options = '--some-option'
+
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('foobar')
+  \     . ' format'
+  \     . ' --some-option',
+  \ },
+  \ ale#fixers#nickel_format#Fix(bufnr(''))