#427 Add a function for looking up ALE variables in buffer scope, and then global scope

autoload/ale.vim

@@ -96,3 +96,14 @@ endfunction
 function! ale#ResetLintFileMarkers() abort
     let s:should_lint_file_for_buffer = {}
 endfunction
+
+" Given a buffer number and a variable name, look for that variable in the
+" buffer scope, then in global scope. If the name does not exist in the global
+" scope, an exception will be thrown.
+"
+" Every variable name will be prefixed with 'ale_'.
+function! ale#Var(buffer, variable_name) abort
+    let l:full_name = 'ale_' . a:variable_name
+
+    return getbufvar(a:buffer, l:full_name, g:[l:full_name])
+endfunction

test/test_ale_var.vader

@@ -0,0 +1,16 @@
+Before:
+  let g:ale_some_variable = 'abc'
+
+After:
+  unlet! g:ale_some_variable
+
+Execute(ale#Var should return global variables):
+  AssertEqual 'abc', ale#Var(bufnr(''), 'some_variable')
+
+Execute(ale#Var should return buffer overrides):
+  let b:ale_some_variable = 'def'
+
+  AssertEqual 'def', ale#Var(bufnr(''), 'some_variable')
+
+Execute(ale#Var should throw exceptions for undefined variables):
+  AssertThrows call ale#Var(bufnr(''), 'undefined_variable_name')