Add GLSL linter using glslang (#914)

* Add a glslang linter for GLSL

doc/ale-glsl.txt

@@ -0,0 +1,36 @@
+===============================================================================
+ALE GLSL Integration                                         *ale-glsl-options*
+                                                         *ale-integration-glsl*
+
+===============================================================================
+Integration Information
+
+  Since Vim does not detect the glsl file types out-of-the-box, you need the
+  runtime files for glsl from here: https://github.com/tikhomirov/vim-glsl
+
+  Note that the current glslang-based linter expects glslangValidator in
+  standard paths. If it's not installed system-wide you can set
+  |g:ale_glsl_glslang_executable| to a specific path.
+
+
+===============================================================================
+glslang                                                      *ale-glsl-glslang*
+
+g:ale_glsl_glslang_executable                   *g:ale_glsl_glslang_executable*
+                                                *b:ale_glsl_glslang_executable*
+  Type: |String|
+  Default: `'glslangValidator'`
+
+  This variable can be changed to change the path to glslangValidator.
+
+
+g:ale_glsl_glslang_options                         *g:ale_glsl_glslang_options*
+                                                   *b:ale_glsl_glslang_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to glslangValidator.
+
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -49,6 +49,8 @@ CONTENTS                                                         *ale-contents*
       gcc.................................|ale-fortran-gcc|
     fusionscript..........................|ale-fuse-options|
       fusion-lint.........................|ale-fuse-fusionlint|
+    glsl..................................|ale-glsl-options|
+      glslang.............................|ale-glsl-glslang|
     go....................................|ale-go-options|
       gometalinter........................|ale-go-gometalinter|
     graphql...............................|ale-graphql-options|
@@ -220,6 +222,7 @@ Notes:
 * Erlang: `erlc`, `SyntaxErl`
 * Fortran: `gcc`
 * FusionScript: `fusion-lint`
+* GLSL: glslang
 * Go: `gofmt`, `go vet`, `golint`, `gometalinter`!!, `go build`!!, `gosimple`, `staticcheck`
 * GraphQL: `gqlint`
 * Haml: `haml-lint`