Implement different log-levels for clearer user feedback

[suvayu]

For example:

• INFO: merging table foo with 101 rows with table bar with 96 rows
• WARNING: column header missing
• ERROR: File not found!
• DEBUG: Applying user function: print function object

[abelsiqueira]

Is this meant to be using Base's @info, and co (https://docs.julialang.org/en/v1/stdlib/Logging/)? It could apply to our other packages as well.

[clizbe]

Not sure of the implementation, but we were discussing how much info the user receives while dealing with these somewhat "invisible" operations (because they're creating temp tables, etc). So for instance if they're merging columns and it's auto-filling empty data with something, it should tell them that. Or warn them their data doesn't exactly match, etc.