Fixed cave render for solid unknown blocks.

is_transparent returns 1 for every unknown block because that's an easy way to
make them ignore lighting, however, this makes cave render useless with a map
containing unknown ores.

The definition is_known_transparent checks whether a block is transparent and is known.

This is a workaround, a proper fix would be to have unknown blocks be neither transparent nor
solid. Or make them both. But don't prefer one of the two.
Merge pull request #988 from CounterPillow/ftb-cave-fix

Fixed cave render for solid unknown blocks.
Added some icons for people to use in their POIs.
Moved icons to subdirectory, adjusted docs

I also added one of the icons to the manualpois example, so people
have an exmaple of how they can use these in their configurations.
Fix error reporting in genpoi when a bad dimension is specified
Update config.rst

Fix typo in example code that causes error in an actual config file.
Item list update
Merge pull request #998 from Disassembler0/master

Item list update
Merge PR #997
Fix additional typos in docs (thanks tomthorogood)
Merge PR #991
Merge remote-tracking branch 'overviewer/master'