Implement metric Tonnes

[guedressel]

Currently "ton" is implemented for the BI (imperial) system. For me as a central european it is common, to use "ton" (symbol "t") for 1,000kg (megagram).

Would be handy to be able to have a distict "Ton" unit for the metric system. (And maybe also a diffentiation between long ton and short ton.)

As reference: https://en.wikipedia.org/wiki/Tonne

[bosix]

Hi :), I would like to see all 3 "tons" implemented as well. Adding the convertion multipliers is simple but I'm struggling on naming things because "ton" is used for "metric ton", "short ton" and "long ton" depending on where you are. Also, we would add a breaking change because the symbol "t" is actually for the metric ton and not as currently set inside this library for the long ton. In a way, you could also see it as a bugfix since the current symbol is incorrect.

According to wikipedia (https://en.wikipedia.org/wiki/Tonne), I suggest the following:

• 1 ton (t) = 1000 kg; alias "metric ton"
• 1 long ton (tn. l.) = 1016.047 kg; alias "imperial ton" or "british ton"
• 1 short ton (tn. sh.) = 907.18 kg; alias "amarican ton"

@bcicen what do you think about it?