Draft: Allow Lua scripts to arbitrarily read a lump.

Adds a new io function: io.openlump(string filename, string mode): open a lump for reading.

• Throws an error if the Lua script is not within a PK3 or WAD.
• Helpful for default configs in PK3s and WADs where the scripter doesn't want to include one directly in the Lua scripts themselves.

NOTE: To accomplish this, internally the lump's data is copied over to a temporary file, which is then given to Lua. This implementation detail can be replaced with a better method down the line without effecting the perceived behavior to Lua scripters.

Despite this being a small change, I don't expect this will make it to 2.2.9 release; it'd be preferable if merging this were delayed to when SRB2 becomes more stable.

In the mean time, curious scripters can checkout and compile the branch themselves.