Node.js v10 support

[mattmorgis]

Should close #17 and #18.

There seemed to be a few ways to go about updating this to work with Node.js v10.

I went through the process of updating level. This required wrapping multileveldown in encoding-down and cause some of the tests to fail with encoding issues. For instance, a test would want 42, but would get '42' or the Buffer for 42, but not the actual int.

An easier path seemed to keep level at v1.x, which handles encodings, and have it use v2 of leveldown. This is what my fork does.

I then updated this to use that new fork, resulting in a successful install on Node.js v10.

[achingbrain]

Hopefully this will get merged but you might be better off renaming your fork & publishing it as a new module.

[vweevers]

Superseded by #20.