Add customizable variables for cheatsheet

[katomuso]

Add multiple customizable variables to control different aspects of cider-cheatsheet and cider-cheatsheet-select behavior.

[katomuso]

@bbatsov Can I be added to the cider-cheatsheet.el package authors?

[bbatsov]

@bbatsov Can I be added to the cider-cheatsheet.el package authors?

Sure.

[katomuso]

@bbatsov Can I be added to the cider-cheatsheet.el package authors?

Sure.

Thank you! I'll update the package commentary in a separate PR after this one is finished.

[katomuso]

I've made all the changes as discussed in the review:

• Renamed cider-cheatsheet-doc-function to cider-cheatsheet-default-action-function and tweaked docstring
• Renamed cider-cheatsheet-select-path to cider-cheatsheet-select-type, tweaked docstring and removed prefix argument for cider-cheatsheet-select
• Removed cider-cheatsheet-select-path-separator and cider-cheatsheet-indentation-function

[bbatsov]

Thanks!