postgresql plugin: Added support for custom queries.

postgresql plugin: Added support for custom queries.

The user may now define and use custom queries to collect data. A query is
defined by specifying the SQL query to execute and a definition of the data
type of each result column:

  <Query magic>
    Query "SELECT magic FROM wizard;"
    Column gauge magic
  </Query>

The "Column" configuration option specifies the type name and optional type
instance: Column <type> [<type_instance>]. The number and order of the
"Column" option has to match the columns of the query result.

A query is activated by adding the configuration option "Query <name>" to the
appropriate "<Database>" configuration blocks. A query may be used multiple
times.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>