Add generic types and ability to mutate files on update and delete operations

[moritzleidinger]

Hi, great library!

I've used it in a small project a bit, but ended up needing the ability to actually mutate the source files when updating or deleting rows. In this PR I've added a parameter (mutateDb) to the add, update and delete methods with default values that keep the current behaviour (mutating for add, non-mutating for update and delete). To make this an atomic operation, I've used your temporaryFileTask to update/delete the values in a temp file and subsequently replace the original file in a single rename operation.

I've also added generic types that can be passed like this: jsonlDir("path").file<DataType>("name"). This provides type safety for returned rows – and falls back to the existing JsonObject type if no type is provided.

I've also added tests to cover update and delete when passing mutateDb=true.

[moritzleidinger]

Hi, just wanted to ask, if you'd consider merging my changes / additions. I'm happy to adapt, if you'd like. Otherwise I'll close the PR and fork. Thanks!

[moritzleidinger]

Thanks!