[i18n] Allow for full customization of all hard-coded strings

[jlmmns]

As of now, only days and months can be customized.

It would be nice if we could customize all hard-coded strings, such as:

• units (seconds, minutes, hours, etc.)
• since (in, almost, ago, etc.)

[spencermountain]

hey @jlmmns yeah you're right - what are you thinking about for i18n units, this?

spacetime.now().add(1, 'journée')

otherwise a dupe of #110

[jlmmns]

@spencermountain Thanks for the quick follow-up! The API should stay English only, I'm sure.

I'm talking about the formatting. So, it's related to #110, indeed. But more than just the "ago" string.
Pretty much every hard-coded string that spacetime displays, should be customizable.

I'd want to try to contribute with a PR, but I don't have any time, unfortunately.

[spencermountain]

merging with #110
cheers