Implement Lua ALE setup & overhaul documentation

1. Add ale.setup and ale.setup.buffer for pure Lua configuration.
2. Update many global settings to use Booleans instead of numbers to
   make types easiert to work with in Lua.
3. Radically reformat documentation and fix errors to make
   documentation more usable for Neovim users.

lua/ale/init.lua

@@ -0,0 +1,46 @@
+local ale = {}
+
+local global_settings = setmetatable({}, {
+    __index = function (_, key)
+        return vim.g['ale_' .. key]
+    end,
+    __newindex = function (_, key, value)
+        vim.g['ale_' .. key] = value
+    end
+})
+
+local buffer_settings = setmetatable({}, {
+    __index = function (_, key)
+        return vim.b['ale_' .. key]
+    end,
+    __newindex = function (_, key, value)
+        vim.b['ale_' .. key] = value
+    end
+})
+
+ale.set_global = function(c)
+    for key, value in pairs(c) do
+        global_settings[key] = value
+    end
+end
+
+ale.set_buffer = function(c)
+    for key, value in pairs(c) do
+        buffer_settings[key] = value
+    end
+end
+
+ale.setup = setmetatable({
+  global = function(c)
+    ale.set_global(c)
+  end,
+  buffer = function(c)
+    ale.set_buffer(c)
+  end,
+}, {
+  __call = function(self, c)
+    self.global(c)
+  end,
+})
+
+return ale