Add a `session_available` method to check if session file exists

[fmbarina]

Hi, just set up persistence.nvim and really liking it so far. I thought it'd be nice to add a quick "restore session" button on my dashboard, and conditionally only show that button when a session exists for the current working directory.

Unsure if I should've written a short example on the README/docs so users would find it easier.

EDIT: I'm so sorry, I just read #30 also adds this feature. I hadn't looked beyond the PR titles. Should I just close this one?