Use consistent initialise logic for modules

[NickColley]

For some components we are initialising all instances of modules, which means if someone wants control over what is initialised they can't do it.

In #616 we split this logic out https://github.com/alphagov/govuk-frontend/pull/616/files#diff-9b2996c4ee664e732889da8297feafd3R12 but there is an inconsistency.

We could consider repeating this pattern for the other components, or consider a wrapper function such as https://github.com/alphagov/govuk_frontend_toolkit/tree/master/javascripts/govuk.

This alongside #660 would make it easy for people to have control over when they want components to initialise, useful if you need to wait for the document to be ready for example.