Introduce diesel_sqlite feature (for rocket_db_pools support)

[mcclure]

Attempt at #2967 . I am pretty uncertain about this and I do not yet know how to test this (aside from the basic smoke test in the PR— see below).

I ran ./scripts/test.sh --contrib as requested in the testing guide. It failed because of #2969. diesel.rs has three tests in it for MysqlPool, PgPool, and (now) SqlitePool, but it appears only the Mysql test is run. I don't know if this is because of #2969 or some other reason.

[mcclure]

Need to do some cleanup before I can post my test case, but I compiled my test app against this PR, called diesel::insert_into on the Connection<> out of the pool, and on quitting out was able to find my inserted row in the Sqlite database.

The patch works.

(I do see something odd which is that when compiling against this Rocket instead of the release version, all the helpful printouts that normally occur are silenced, and if there is a failure such as a bad address in Rocket.toml $? after executing is 0 instead of 110 as it is in v0.5.1. However I see these same results even if I don't attach my database, so I think this is a difference between v0.5.1 and git master and not caused by this patch.)

[mcclure]

By merging this PR with #2972 , I can now run the tests. They complete successfully and at one point prints

:: Building and testing sync_db_pools [diesel_sqlite_pool]...

So I believe the test embedded in this PR is also running successfully. I am continuing to develop my application against this PR branch, still without [database-related] problems.