turn relative paths for out_dir and tmp_dir into absolute paths

[Turmfalke2]

two blemishs:

• the translation from relative to absolute path happens in 'is_directory' which may be confusing/unexpect. Might be able to do it with an extra listener? Unsure where to put that one
• upon setting a value with relative path it still outputs the old relative value, even thought it sets the absolute one. Fetching the new one is problematic incase the variable requires setup.

Fixes #14

turmfalke@Industria ~/PyPoE_work $ pypoe_exporter config set out_dir ./out
22:46:23 Config setting "out_dir" has been set to:
./out
turmfalke@Industria ~/PyPoE_work $ pypoe_exporter config get out_dir
22:46:27 Config setting "out_dir" is currently set to:
/home/turmfalke/PyPoE_work/out