Add bits option

[montyanderson]

Switch units based on boolean 'bits' flag.

[sindresorhus]

What’s the use-case? Would be good to include that in the docs for the option.

[super3]

@sindresorhus Download/upload speed is usually represented in bits/s vs bytes/s. We are using this for logging on YaCDN:

{ n: 14291272,
4|yacdn-5  |   url:
4|yacdn-5  |    'http://meowbaari.com/wp-content/uploads/2016/06/1464933654_cat_sleep.png',
4|yacdn-5  |   referer: undefined,
4|yacdn-5  |   size: '3.05 kB',
4|yacdn-5  |   time: '2101ms',
4|yacdn-5  |   *speed: '1.45 kbit/s'* }

[sindresorhus]

Download/upload speed is usually represented in bits/s vs bytes/s. We are using this for logging on YaCDN:

👍 Makes sense. Would be good to include the use-case about speed in the docs for the option.

[super3]

@sindresorhus Are the changes satisfactory?