Macrons for some long vowels

[blagarde]

The Hepburn romaniser seems to follow the Modified Hepburn version.
http://en.wikipedia.org/wiki/Hepburn_romanization

e.g. It produces 'shinpai' instead of 'shimpai'.

Modified Hepburn has complex rules for placing macrons to indicate that some vowels are long, which the romaniser does not currently do.

e.g.
Expected results

ちゅうい  chūi
みずうみ  mizuumi

Actual results:

ちゅうい  chuui
みずうみ  mizuumi

Another example is with 'tōkyō', which currently gets output as 'toukyou'.