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_var_spec.lua

@@ -0,0 +1,51 @@
+local eq = assert.are.same
+local ale = require("ale")
+
+describe("ale.var", function()
+    local buffer_map
+
+    setup(function()
+        _G.vim = {
+            api = {
+                nvim_buf_get_var = function(buffer, key)
+                    local buffer_table = buffer_map[buffer] or {}
+                    local value = buffer_table[key]
+
+                    if value == nil then
+                        error(key .. " is missing")
+                    end
+
+                    return value
+                end,
+            },
+            g = {
+            },
+        }
+    end)
+
+    teardown(function()
+        _G.vim = nil
+    end)
+
+    before_each(function()
+        buffer_map = {}
+        _G.vim.g = {}
+    end)
+
+    it("should return nil for undefined variables", function()
+        eq(nil, ale.var(1, "foo"))
+    end)
+
+    it("should return buffer-local values, if set", function()
+        _G.vim.g.ale_foo = "global-value"
+        buffer_map[1] = {ale_foo = "buffer-value"}
+
+        eq("buffer-value", ale.var(1, "foo"))
+    end)
+
+    it("should return global values, if set", function()
+        _G.vim.g.ale_foo = "global-value"
+
+        eq("global-value", ale.var(1, "foo"))
+    end)
+end)