Notifyfunc impl and add mount subtype

[inevity]

impl fuse notify func
add fuse subtype mountconfig

[jacobsa]

Could you please

1. Add some description of what this is, with background context about why you'd want it and how it works, to both the commits and the pull requests?

2. Add some documentation to the code, for at least the functions in package fuse?

At a glance I have no idea what this is.

(It also sounds like the two commits are for two orthogonal things? If so, please send two pull requests.)

[inevity]

About subtype ,see pull request #34

[inevity]

About impl fuse notify func ,see pull request #35

[stapelberg]

[helping out with maintenance, doing first round of pull request triage]
Closing this one in favor of #34 and #35