Support MongoDB collations

[4strid]

This is the best way to perform case insensitive search, and should be supported.

• Add a method to the cursor for collation (maybe make the Cursor a proxy of the underlying cursor? Then we just support every MongoDB feature out of the box)
• Add a way to set a default collation for a given collection (?)
• Add warning methods to NeDB client which explain that NeDB does not support collation
• Add a note to the README about collation indexes (maybe start a set of "examples" and show using collation to perform case insensitive search in its own example)