Changed 'vars' arg in lume.format() to be optional

README.md

@@ -211,7 +211,7 @@ instead of whitespace.
 lume.trim("  Hello  ") -- Returns "Hello"
 ```
 
-### lume.format(str, vars)
+### lume.format(str [, vars])
 Returns a formatted string. The values of keys in the table `vars` can be
 inserted into the string by using the form `"{key}"` in `str`.
 ```lua

lume.lua

@@ -231,6 +231,7 @@ end
 
 
 function lume.format(str, vars)
+  vars = vars or {}
   local f = function(x) return tostring(vars[x] or "{" .. x .. "}") end
   return (str:gsub("{(.-)}", f))
 end