Optional options parameter

[nickfaughey]

Added an extra optional parameter to the main formatSI function to allow 3 extra options:

• space: adds a space between number and prefix (ex. 1.2 k)
• plusMinus: prepends a +/- sign (ex. +1.2k)
• suffix: appends a string suffix for units (ex. 1.2kWh)

[ThomWright]

Hi, thanks for the PR.

Having skimmed the changes, they look good, and they include docs changes which is great.

Would you be able to add some tests?

Also, for the most part, IIRC (it's been a while) this library is supposed to be largely bidirectional except for some unavoidable edge cases. That is, the output of formatSI should be able to be reversed by unformatSI (and vice versa). Do you think it would be possible to implement the corresponding unformatSI changes?

Finally, do you mind if I ask what you're using this for? It's been so long since I thought about this library, I'm almost surprised anybody is using it!

[nickfaughey]

Certainly, I'll add tests and add the reverse unformatSI functionality (forgot about that one). I stumbled across this library when I wanted a simple implementation of a 1000 -> k, 1000000 -> M function where I'm displaying total energy consumption on a dashboard that could float anywhere from single-digit watts to megawatts. Discovered that 1.23 kWh looks better than 1.23kWh which sparked the original PR, then added the suffix and prefix to easily have a delta display, ex. +3.4 kWh vs last week.