support custom fs

[joehand]

Adds support for custom fs. Does not break current API.

var db = toiletdb({name: 'file.json', fs: archive})

// db.write etc

I wasn't sure about if we wanted to include the writing of a temporary file for the custom fs. For example, it doesn't seem like in hyperdrive we'd want to write a temporary file. Thoughts?

[joehand]

I wasn't sure about if we wanted to include the writing of a temporary file for the custom fs. For example, it doesn't seem like in hyperdrive we'd want to write a temporary file. Thoughts?

Made it to not write temp files for a custom fs. We can add this as an option later.