Proposal: sub-packages to enable optional features

[emersion]

A common pain point when using go-sqlite3 is that enabling optional features requires adding a build tag to all go command invocations. For instance, when a program depends on FTS5, it's no longer possible to run go build, go test, etc as usual. End users need to explicitly add -tags sqlite_fts5 every time.

Here is a proposal to improve this. A new fts5 sub-package is introduce. Any program which depends on FTS5 can import it to turn on FTS5:

import _ "github.com/mattn/go-sqlite3/fts5"

What do you think?