Add support for localisation via JavaScript configuration to Accordion component

[querkmachine]

This is related to two issues, #2802 and #2698, as I couldn't find a way to cleanly separate them.

• Modifies the initAll function to pass options.accordion into the accordion initialisation.
• Modifies the accordion component to accept a second config parameter (defaults to an empty object).
• Modifies the accordion component's config merging to accept the config parameter.
• Updates the 'translated' full-page example to call initAll with a custom configuration.
• Adds integration tests checking that the accordion's text is modified according to the configuration.
• Adds unit tests to the existing mergeConfig function to ensure that it supports empty objects.

Todo

#2802 includes a task to ensure that "Data attribute translations take priority over JS translations, if both provided"

This is partially covered by the configuration merging function's unit tests, though that function also doesn't make an explicit distinction between configuration passed from data-* attributes and those from JavaScript. Do we feel like we need to explicitly test for parameter ordering?

[36degrees]

LGTM! 👍🏻