Fix performance bug caused by excessive computation when outputing to…

… a file

Document generalized aggregate expressions

Add documentation about the where operator

Update README

Bump version

Small bugfixes

- Fix padding when formatting aggregates
- Exit with a status code of 1 when the query is invalid

Add fields operator + big performance improvement for aggregates

Add fields operator

Rename to agrind

Performance improvements + support for `parse from` & `json from`