Add a default hint for the multiselect prompt

[jessarcher]

This PR uses the hint feature introduced in #44 to add a default hint to the multiselect prompt, instructing users to use the space bar to select options. I went with the term "space bar" as it is the dominant wording used on Wikipedia.

A few people have raised this issue specifically referring to the multi-select prompt used in the Laravel installer, which will be many people's first exposure to Prompts and Laravel. We could just add this hint to the multi-select prompts in the installer, but ultimately it seemed more user-friendly to make it the default. If someone does not want the hint, they would need to pass an empty string to the hint argument.

Fixes #41
Fixes #48

[shane-smith]

This is great, thank you @jessarcher!

As the author of #41, I agree this solves the problem I ran into.