Escape path queries for SQLite

[sampsyo]

Path queries in beets use the SQLite LIKE operator to search subdirectories. For example, beet ls /foo generates an SQL expression like path LIKE '/foo/%'. This can fail when the path contains characters that SQLite wants to interpret specially. Specifically, if there's an underscore in the path, SQLite matches any character in that position.

We should escape _ and % in these strings before sending them to SQLite.