Feature request: Allow for configuring `max_write`

[akhi3030]

Currently, the value of max_write is hardcoded in https://github.com/zargony/fuse-rs/blob/master/src/request.rs#L93. This is a bit problematic for my usecase where I want to be able to create file sytems with different max_write` parameters.

I have a patch which I will submit as a PR shortly that implements the above. However, the PR changes the API of mount and spawn_mount. Given the existing API, I wasn't sure how else to introduce this feature. Feedback is welcome.