Add basic Lua ALE functions and test coverage

Ensure that basic ALE functions `ale.var`, `ale.escape`, and `ale.env`
are available in Lua. Cover all Lua code so far with busted tests,
fixing bugs where ALE variables can be set with Boolean values instead
of numbers. Document all functionality so far.

test/lua/ale_env_spec.lua

@@ -0,0 +1,56 @@
+local eq = assert.are.same
+local ale = require("ale")
+
+describe("ale.env", function()
+    local is_win32 = false
+
+    setup(function()
+        _G.vim = {
+            o = setmetatable({}, {
+                __index = function(_, key)
+                    if key == "shell" then
+                        if is_win32 then
+                            return "cmd.exe"
+                        end
+
+                        return "bash"
+                    end
+
+                    return nil
+                end
+            }),
+            fn = {
+                has = function(feature)
+                    return feature == "win32" and is_win32
+                end,
+                -- Mock a very poor version of shellescape() for Unix
+                -- This shouldn't be called for Windows
+                shellescape = function(str)
+                    return "'" .. str .. "'"
+                end,
+                fnamemodify = function(shell, _)
+                    return shell
+                end
+            }
+        }
+    end)
+
+    teardown(function()
+        _G.vim = nil
+    end)
+
+    before_each(function()
+        is_win32 = false
+    end)
+
+    it("should escape values correctly on Unix", function()
+        eq("name='xxx' ", ale.env('name', 'xxx'))
+        eq("name='foo bar' ", ale.env('name', 'foo bar'))
+    end)
+
+    it("should escape values correctly on Windows", function()
+        is_win32 = true
+        eq('set name=xxx && ', ale.env('name', 'xxx'))
+        eq('set "name=foo bar" && ', ale.env('name', 'foo bar'))
+    end)
+end)