Add glslls (#1179)

* Add glslls-based LSP linter
* Make logfile configureable

test/command_callback/test_glslls_command_callback.vader

@@ -0,0 +1,37 @@
+Before:
+  Save g:ale_glsl_glslls_executable
+  Save g:ale_glsl_glslls_logfile
+
+  unlet! g:ale_glsl_glslls_executable
+  unlet! g:ale_glsl_glslls_logfile
+
+  runtime ale_linters/glsl/glslls.vim
+  call ale#test#SetDirectory('/testplugin/test/command_callback')
+
+After:
+  Restore
+  call ale#linter#Reset()
+
+Execute(Executable should default to 'glslls'):
+  AssertEqual
+  \ 'glslls',
+  \ ale_linters#glsl#glslls#GetExecutable(bufnr(''))
+
+Execute(Executable should be configurable):
+  let g:ale_glsl_glslls_executable = 'foobar'
+  AssertEqual
+  \ 'foobar',
+  \ ale_linters#glsl#glslls#GetExecutable(bufnr(''))
+
+Execute(Command should use executable):
+  let command1 = ale_linters#glsl#glslls#GetCommand(bufnr(''))
+  AssertEqual command1, ale#Escape('glslls') . ' --stdin'
+
+  let g:ale_glsl_glslls_executable = 'foobar'
+  let command2 = ale_linters#glsl#glslls#GetCommand(bufnr(''))
+  AssertEqual command2, ale#Escape('foobar') . ' --stdin'
+
+Execute(Setting logfile should work):
+  let g:ale_glsl_glslls_logfile = '/tmp/test.log'
+  let command = ale_linters#glsl#glslls#GetCommand(bufnr(''))
+  AssertEqual command, ale#Escape('glslls') . ' --verbose -l /tmp/test.log --stdin'