[feature] partial restore

[gsmolk]

It should be allowed to restore only specified by name databases or ignore them.
We could use PostgreSQL recovery lax behavior toward missing files and blocks for this.

[gsmolk]

Proposed options: --db-include=datname, --db-exclude=datname

1. Each type can be specified multiple times.
2. Cannot be used simultaneously.
3. 'template0' and 'template1' are always restored.
4. After restore user must manually drop excluded databases.

[gsmolk]

Branch https://github.com/postgrespro/pg_probackup/tree/issue_79