Add support for other words ("decade", "fortnight", "century"...)

[noviluni]

There are some words that can be added to the library:

• Decade: 10 years (2 decade --> 20 years, ...)
• Fortnight: 14 days (Example: "a fortnight ago")
• Century: 100 years (Example: "a century ago", "two centuries ago")

It's not necessary to put them all in a unique PR, you can submit just one of them. Just take in mind that it should support "next decade", "last decade", "a decade ago", "in a decade", but also "two decades ago".

[Gallaecio]

We should see if we can get some of these words from CLDR data.

[seric17]

Hi,
If no one else is already doing it, I'll try to add support for "decade".

[seric17]

In my pull request #762, I made the assumption that "next decade" and "in a decade" both refer to "in 10 years". Similarly, I assumed that "last decade" and "a decade ago" both refer to "10 years ago". Please let me know if this makes sense and if there is anything else I should do. @noviluni

[noviluni]

Hi @seric17, thank you for submitting this PR, I will check it when having a moment 🙂