Add a `pgroll latest url` command to the CLI. The command returns a
Postgres connection string with the `search_path` option set to the
latest version schema name. This can be used to set the connection
string for an application to work with the latest schema version.

By default, with no arguments, `pgroll latest url` will use the
connection string set via the global `--postgres-url` (or
`PGROLL_PG_URL` env var). If an argument is given to `pgroll latest
url`, the command will use that as the base connection string instead.

The resulting connection string sets the `search_path` by setting the
`options` parameter in the URL query string:

```
options=-c search_path='public_01_some_migration'
```

As described in the Postgres [libpq
documentation](https://www.postgresql.org/docs/current/libpq-connect.html).

## Examples

With no connection string argument, use the connection string used by
`pgroll` itself:

```bash
$ export PGROLL_PG_URL=postgres://postgres:postgres@localhost:5432?sslmode=disable
$ pgroll latest url

postgres://postgres:postgres@localhost:5432?options=-c%20search_path%3Dpublic_05_some_migration&sslmode=disable
```

Take the connection string as a command line argument:

```bash
$ pgroll latest url postgres://me:mypass@example.com:5432

postgres://me:mypass@example.com:5432?options=-c%20search_path%3Dpublic_05_some_migration
```

Like `pgroll latest migration` and `pgroll latest schema`, `pgroll
latest url` takes a `--local/-l` flag to take the latest version schema
name from a directory of migration files rather than the target
database. This allows for a connection string to be constructed offline,
without having to lookup the latest version schema name in the target
database.

```bash
$ pgroll latest url --local migrations/ postgres://me:mypass@localhost:5432

postgres://me:mypass@localhost:5432?options=-c%20search_path%3Dpublic_05_some_migration
```

The resulting connection string can be used directly as an argument to
tools like `psql` that accept a connection string on the command line:

```bash
psql $(pgroll . latest url)
```

Documentation for the new command is added in
#928

Closes #900