default/alternative values in variables

[Julian]

Is your feature request related to a problem? Please describe.
it could come handy to have default or/and alternative values in variables.

Describe the solution you'd like

{{imageNameKey|dirName|my_default_value}}-{{fileName}}

where the resulting iamge filename would be

• image_name_key-file_name (imageNameKey is set, dirName can be set but don't have to)
• dir_name-file_name (imageNameKey is not set and dirName is set)
• my_default_value-file_name (imageKeyName and dirName are not set)

kind of related to #44