Add a mechanism for more complex transformations of parsed output.
This is a **breaking change**: it renames the existing `add_transform` method to `add_lookup` (for single-field lookups). The new `add_transform` method allows modifying many fields at once, or even replacing the parsed object completely. (it is useful for, eg, annotating 3 fields at once, or writing an adapter that cleans up the existing values in a task-specific way- like converting between `chr1` and `1` for chromosome names)
We generally recommend using lookups for most cases; they are easier to write and test as individual pieces of code. Transforms allow replacing the entire parsed contents of the row, but they also make it much easier to introduce bugs.