Allow downstream crates to decide on how rusqlite should be built

[mehcode]

As cargo features are additive, using migrant_lib in a crate currently forces the crate to use bundled rusqlite.

[jaemk]

Good idea -- Would you mind adding a note under the "Available Features" table in src/lib.rs to let users know that if they want the "sqlite-driver" (in this case, rusqlite) to be bundled with the executable, they need to add rusqlite as a separate dependency with the bundled feature - even if they aren't using rusqlite directly themselves? (and then use readme.sh to regen the README) << I guess this only matters for crates like migrant which don't actually use rusqlite, but still want sqlite bundled, so I guess the note isn't really necessary.