Support negative numbers

[sindresorhus]

require('filesize')(-4) returns an empty string. Should return -4.00B.

[avoidwork]

What's the use case where this is valid? I'm unaware of any file system allowing for smaller than 0B files.

[sindresorhus]

If you want to show the size difference of something.

Example: https://github.com/sindresorhus/grunt-sizediff