feat: add variable for extension

[SeanMcP]

I like this system but prefer to write notes in markdown. This PR adds a variable for NOTES_EXTENSION that defaults to txt. With these changes, a user can choose what type of file they would like to create with the script while preserving the current behavior.

[nickjj]

Hi,

Just replying to acknowledge the PR for now.

I like the idea in general, but I think we'll need to change some of the wording in the readme file. For example the part you modified was related to customizing the notes path but this extension variable doesn't really fall in line with the path. This will likely end up introducing a new customization section in the docs that goes over each variable independently.

Let me think about it for the next day or 2 and I'll get back to you on the exact wording for the readme then we'll merge it in. Also if you want to take a shot at the new wording, go for it. I mainly need a couple of days because my schedule is slammed.