Feature: configurable custom file exports

[KarolakJakub]

This PR includes a feature that allows to create custom export options, as described in configuration.md - users will be able to add multiple configurable export types with different formatting, locales etc. If no defined exports are found within the config, Turnilo will use the default 'Export to TSV' and 'Export to CSV' ones.

[adrianmroz]

Let's extract bug fix to another PR, so we could merge it earlier. Rest of it should wait till we discuss what is needed and how we should achieve it. For reference #504

[KarolakJakub]

Bugfix extracted to https://github.com/allegro/turnilo/pull/536