Add postgesql collectd plugin support

[anryko]

Add postgesql collectd plugin support

[anryko]

I realised that my postgresql collectd config is uniq and no one will ever reuse my getdash plugin. So I'll just close this issue... :)

[anryko]

I'll add support for:

The following predefined queries are available (the definitions can be found in the postgresql_default.conf file which, by default, is available at prefix/share/collectd/):
backends
    This query collects the number of backends, i. e. the number of connected clients.
transactions
    This query collects the numbers of committed and rolled-back transactions of the user tables.
queries
    This query collects the numbers of various table modifications (i. e. insertions, updates, deletions) of the user tables.
query_plans
    This query collects the numbers of various table scans and returned tuples of the user tables.
table_states
    This query collects the numbers of live and dead rows in the user tables.
disk_io
    This query collects disk block access counts for user tables.
disk_usage
    This query collects the on-disk size of the database in bytes.

[anryko]

Done 16fba9a