Convert to a proper minor mode

[davidshepherd7]

Hi,

I think sql-indent should be a minor mode (it's uncommon to have code which does something merely by loading it). However changing it to a minor mode now will break peoples configs, which is probably bad, thoughts?

We could also add a custom variable (defaulting to t) and add the hook automatically as it does at the moment if it is true. This would avoid breaking existing configs, but is a bit of a hack.

[bsvingen]

I think that's fine, though the alternative would be to fork an sql-indent-2, where changes could be made without restraint.

[purcell]

FWIW, I think progressive/modernisation changes like this PR should be merged, even if it changes the behaviour for some users. Otherwise, what's the point of maintaining this here? To be complete, of course, changes should update the docs as well as the code. You can conservatively tag certain versions of the code with version numbers in order to make "stable" versions appear in MELPA Stable, but otherwise, I think development should march onwards, and users will generally be willing to receive even slightly breaking improvements.