fixed: EXR compression options not exposed

EXR compression property was not exposed when switching to OpenEXR in
the img_format dropdown
add: support for variables in output path + filename variable

Added support for using variables in the output path.
Also added support for the %F variable (blend filename).
Merge pull request #118 from Mindtransplant/master

fix: EXR compression, add: variables support in filepath