Improve pyrex cython linter.

Like many other linters, use variables for the executable and options
used by the linter.
By default, the linter now report every warnings as errors with
`--warning-errors`.
Also add include directory and set working directory to file directory.

doc/ale-pyrex.txt

@@ -0,0 +1,25 @@
+===============================================================================
+ALE Pyrex (Cython) Integration                              *ale-pyrex-options*
+
+
+===============================================================================
+cython                                                       *ale-pyrex-cython*
+
+g:ale_pyrex_cython_executable                   *g:ale_pyrex_cython_executable*
+                                                *b:ale_pyrex_cython_executable*
+  Type: |String|
+  Default: `'cython'`
+
+  This variable can be changed to use a different executable for cython.
+
+
+g:ale_pyrex_cython_options                         *g:ale_pyrex_cython_options*
+                                                   *b:ale_pyrex_cython_options*
+  Type: |String|
+  Default: `'--warning-extra --warning-errors'`
+
+  This variable can be changed to modify flags given to cython.
+
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -185,6 +185,8 @@ CONTENTS                                                         *ale-contents*
       puglint.............................|ale-pug-puglint|
     puppet................................|ale-puppet-options|
       puppetlint..........................|ale-puppet-puppetlint|
+    pyrex (cython)........................|ale-pyrex-options|
+      cython..............................|ale-pyrex-cython|
     python................................|ale-python-options|
       autopep8............................|ale-python-autopep8|
       black...............................|ale-python-black|