Add rpm spec file linter (rpmlint) (#486)

* Initial attempt at an rpmlint linter.

* Add some basic documentation.

* Play with indentation in the test file.

* Another attempt to fix the rpmlint test.

* Hopefully this does it.

doc/ale-spec.txt

@@ -0,0 +1,28 @@
+===============================================================================
+ALE RPM Spec Integration                                     *ale-spec-options*
+
+
+-------------------------------------------------------------------------------
+rpmlint                                                      *ale-spec-rpmlint*
+
+g:ale_spec_rpmlint_executable                 *g:ale_spec_rpmlint_executable*
+
+  Type: |String|
+  Default: `'rpmlint'`
+
+  This variable sets executable used for rpmlint.
+
+
+g:ale_spec_rpmlint_options                       *g:ale_spec_rpmlint_options*
+
+  Type: |String|
+  Default: `''`
+
+  Set this to pass extra arguments to rpmlint.
+
+  For example, to instruct rpmlint to use a specific configuration file:
+
+  let g:ale_spec_rpmlint_options = '-f custom.cf'
+
+-------------------------------------------------------------------------------
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: