Convert Japanese calender year notation to western calendar year notation

[melink14]

Is stuff like this possible?
昭和58 -> 1983
昭和五十八年 -> 1983
大正元年 -> 1912

Looks like there is a different chrome extension that does this, but would be helpful to have it in rikaikun since I have it on a lot anyway!

Originally posted by @zmcnulty in #1050

[tora-pan]

Do we also want to do the reverse?
1983 -> 昭和58 & 昭和五十八年
1912 -> 大正元年

[zmcnulty]

In my experience, reverse conversion is usually only necessary when dealing with Japanese beauracracy, i.e. the world of paper forms and fax machines. Probably not necessary for a browser extension.