tables plugin: Added a generic plugin to parse tabular data.

tables plugin: Added a generic plugin to parse tabular data.

Currently, the main purpose of this plugin is to be able to get information
from the Linux proc filesystem but it should be flexible enough to get data
from other sources as well.

Values are selected based on a given column separator and column numbers. The
configuration is a kind of a mix of the tail and *sql plugins' configurations.
A sample configuration might look like this:

  <Plugin table>
    <Table "/proc/slabinfo">
      Instance "slabinfo"
      Separator " "
      <Result>
        Type gauge
        InstancePrefix "active_objs"
        InstancesFrom 0
        ValuesFrom 1
      </Result>
      <Result>
        Type gauge
        InstancePrefix "objperslab"
        InstancesFrom 0
        ValuesFrom 4
      </Result>
    </Table>
  </Plugin>