fix: allow functions that return strings for output

[timteeee]

When copying the suggested config in the readme, I found a bug with the open_by_os function where it was passing the output to vim.fn.expand which expects a string. The suggested config in the readme is actually a function that returns a string, resulting in the following error:

This fix allows for output to be either a string or a function that returns a string, in order to support the suggested config in the readme.

[isabelroses]

Thanks for the fix