added valuesabsolute and valuespercentage to df plugin

In collectd 5.4.0, the df plugin was updated to include ValuesAbsolute and ValuesPercentage options.
This commit adds these to the df.pp and the df.conf.erb files.  The default values mirror what is in the source code for the df plugin.
I'm not sure if this is backwards compatible for previous versions of collectd, so I'll let you decide whether to accept the PR.
If there are any changes you require to accept the PR, please let me know.