ColumnItems are now virtualized

[matt-aitken]

Only the Column View items on screen get rendered, using react-virtual.

This doesn't fix all the issues with very large JSON files, though it is a step in the right direction.

Related to issues #42 #18.

Remaining issues, to be fixed in a separate branch after this is merged:

Locally I'm getting RangeError: Maximum call stack size exceeded errors for very large files. I've found two separate causes for this:

1. Getting related paths, i.e. the getRelatedPathsAtPath function. Example: https://jsonplaceholder.typicode.com/photos
2. Inferring the types. Example from @ubuntudroid above: https://raw.githubusercontent.com/kareltucek/firmware/master/examples/mirroring_keymap.json