Facilitate code-splitting

[Mesoptier]

See #67

I thought this PR would be easy, but there are some issues:

1. Since the current build setup makes it difficult to compile code that is suitable for tree-shaking/code-splitting, I decided to suggest importing the shouldShowPrompt function directly from the src/ directory instead. Not ideal, but it might be OK.
2. The shouldShowPrompt function will be called twice: once by the developer to determine whether to import the rest of the library, and once by the library to determine whether to show the prompt. This would be fine, except the function has side-effects: it increments the visits counter on each call, meaning that the prompt might be shown too soon or not at all. This is breaking.

What are your thoughts on this, @chrisdancee?