The idea is that this command allows the manipulation of PostgreSQL alias
names, stored as links into an `/alias` directory in the `PGENV_ROOT` directory.
Subcommands `add` and `remove` checks for the existance of the links
and create or delete such links int he alias directory.

Changes to the `versions` command in order to show also the aliases.
Now, after the directory output for each version, all aliases are printed out.
This changes the format of the `versions` output.

This way a user can specify a version to use/remove as an alias, and
the script will produce (normalize) the right version to use for disk layout.

Added a function to remove all the aliases of a version when deleting
an installation.

THIS BREAKS BACKWARD COMPATIBILITY.

Now each instance is installed into a directory that includes
a timestamp and is collected into a directory named by the PostgreSQL version,
for instance
 $PGENV_ROOT/
      pgsql-12.0/
              pgsql-12.0-2019-10-21-14-15-35
              pgsql-12.0-2019-10-21-08-36-35
     pgsql-11.5/
              pgsql-11.5-2019-10-20-14-15-35
              pgsql-11.5-2019-10-20-08-36-35

and each instance is automatically assigned two aliases:
- one with the version (12.0 for example)
- one with the full timestamp (pgsql-12.0-2019-10-21-14-15-35).

This means that the alias with the single PostgreSQL version (e.g., 12.0)
works only for the first instance installed.

All other commands have been refactored to use the alias as input.

Remove sed dependency in pgenv_versions.

Fix pgenv_versions to allow '*' on current instance.

The `alias remove` prevents the user to lock out removing all the aliases
of an instance.
The `add` command allows for alias creation.