Support VFS when opening a db

[guenhter]

Hi,

is there a reason why it is currently not supported to define a VFS via the connection string when opening a DB?

I guess everything is prepared for it but the zVfs parameter is just not use and always set to nil

go-sqlite3/sqlite3.go

Line 1377 in 92f580b

nil)

Would you mind to add the vfs to the connection uri parameters, so that e.g. this can be done:

db, err := sql.Open("sqlite3", "test.db?cache=shared&mode=rwc&vfs=unix-dotfile")

(I guess as a start, it is good enough only supporting the build-in-VFS'; registering new VFS' can be added if needed later)

[mattn]

Could you please try #877 ?