Support configs that call setup using dot syntax

Resolves (#90)

README.md

@@ -32,7 +32,7 @@ Require the plugin and call `setup` with a config table with one or more of the
 
 ```lua
 -- The setup config table shows all available config options with their default values:
-require("presence"):setup({
+require("presence").setup({
     -- General options
     auto_update         = true,                       -- Update activity based on autocmd events (if `false`, map or manually execute `:lua package.loaded.presence:update()`)
     neovim_image_text   = "The One True Text Editor", -- Text displayed when hovered over the Neovim image

lua/presence/init.lua

@@ -68,7 +68,17 @@ local default_file_assets = require("presence.file_assets")
 local plugin_managers = require("presence.plugin_managers")
 local Discord = require("presence.discord")
 
-function Presence:setup(options)
+function Presence:setup(...)
+    -- Support setup invocation via both dot and colon syntax.
+    -- To maintain backwards compatibility, colon syntax will still
+    -- be supported, but dot syntax should be recommended.
+    local args = {...}
+    local options = args[1]
+    if #args == 0 then
+        options = self
+        self = Presence
+    end
+
     options = options or {}
     self.options = options