Delete target backup

[usernamedt]

Add functionality to delete target backup for Postgres

User can specify the target for deletion via backup name or user data:

wal-g delete target base_0000000100000000000000C4 // delete base backup specified by name
wal-g delete target --target-user-data "{ \"x\": [3], \"y\": 4 }"	//delete backup specified by user data

When deleting full backup, all dependant increment backups will be deleted too.

Deleting the incremental backup is disabled by default, but can be enabled via --find-full flag:

wal-g delete target base_0000000100000000000000C9_D_0000000100000000000000C4 --find-full

In this case, wal-g will search for the base backup and will delete the base backup and all dependant increments.

[g0djan]

LGTM