Detect and use CM files for smlnj (#884)

* Detect and use CM files for smlnj

* Split into two checkers

- one for CM projects
- one for single SML files

* Fix some typos

* Fix error caught by writing tests

We want to actually use `glob` to search in paths upwards from us.
(Previously we were just searching in the current directory every time!)

* Fix errors from former test run

* Write tests for GetCmFile and GetExecutableSmlnj

* Typo in 'smlnj/' fixture filenames

doc/ale-sml.txt

@@ -0,0 +1,36 @@
+===============================================================================
+ALE SML Integration                                           *ale-sml-options*
+
+===============================================================================
+smlnj                                                           *ale-sml-smlnj*
+                                                             *ale-sml-smlnj-cm*
+
+There are two SML/NJ powered checkers:
+
+- one using Compilation Manager that works on whole projects, but requires you
+  to save before errors show up
+- one using the SML/NJ REPL that works as you change the text, but might fail
+  if your project can only be built with CM.
+
+We dynamically select which one to use based whether we find a `*.cm` file at
+or above the directory of the file being checked. Only one checker (`smlnj`,
+`smlnj-cm`) will be enabled at a time.
+
+-------------------------------------------------------------------------------
+
+g:ale_sml_smlnj_cm_file                               *g:ale_sml_smlnj_cm_file*
+                                                      *b:ale_sml_smlnj_cm_file*
+  Type: |String|
+  Default: `'*.cm'`
+
+  By default, ALE will look for a `*.cm` file in your current directory,
+  searching upwards. It stops when it finds at least one `*.cm` file (taking
+  the first file if there are more than one).
+
+  Change this option (in the buffer or global scope) to control how ALE finds
+  CM files. For example, to always search for a CM file named `sandbox.cm`:
+>
+      let g:ale_sml_smlnj_cm_file = 'sandbox.cm'
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -119,6 +119,8 @@ CONTENTS                                                         *ale-contents*
     sh....................................|ale-sh-options|
       shell...............................|ale-sh-shell|
       shellcheck..........................|ale-sh-shellcheck|
+    sml...................................|ale-sml-options|
+      smlnj...............................|ale-sml-smlnj|
     spec..................................|ale-spec-options|
       rpmlint.............................|ale-spec-rpmlint|
     stylus................................|ale-stylus-options|