execute(): support passing parameters as keyword arguments

[alyst]

This PR complements JuliaDatabases/SQLite.jl#223 by adding keyarg support to DBInterface (for execute(), executemultiple() and executemany()).

I've also added two other minor things:

• @sql_str macro, so that SQL queries could be written as @sql"....". This should allow embedded SQL highlighting in Julia scripts by IDE plugins as it is done for e.g. @r_str strings of RCall.jl.
• execute(f, db, sql), which (as open(f, file)) allows applying f to the returned cursor and then closing the query (the query is also closed if f throws). This pattern allows tighter control of the prepared statement lifetime, which does not rely on GC to finalize and close the statement object. It also allows DB-specific implementations that don't create prepared statements at all.

[alyst]

@quinnj could you please take a look?