External Lua Library Support

While using Selene, I realized it only had one option for setting the active libraries for Lua scripts, in constructor of sel::State. 

In some cases (modding ability in game dev for example), you don't want Lua to have access to some libraries (io, os, etc). 

So, I came up with a new function for sel::State that does the job. It essentially just ends up being a wrapper due to the lua_State being private.

The parameters are:
modname - the Lua name of the library (the "table" part of "table.insert()")
openf - the function Lua uses to open the library. 

There is nothing to return as [luaL_requiref](http://www.lua.org/manual/5.2/manual.html#luaL_requiref) doesn't return anything. 

The function adheres to the [Lua manuals directions](http://www.lua.org/manual/5.2/manual.html#6).

If you would like test cases, I can give them to you.