added bigendian behavior

[kuri65536]

Thank you for this software, I always use this from wavedrom.

I want to use bigendian like as #22 too.
I made it with small hacking, please pull up.

test results are now:

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	94.74	89	95.83	94.48
index.js	100	100	100	100
render.js	94.67	89	95.83	94.41	... 95,122,123,269

[drom]

Very cool. What is going on with numbers on the top? Are they supposed to be shifted?

bigendian1 is fine: https://gist.github.com/drom/b2371ec90f5340cd4652e215df85814b
bigendian2 & bigendian3: have index labels crooked:
https://gist.github.com/drom/291d501bf1f8e8f89e36c92e28b2e90e
https://gist.github.com/drom/9418c706a979c8d3789b80d44b45fae5

[kuri65536]

Noone supporsed 'crooked labels'.
I want to describe byte data by this software, and added a bitscale otpion.
I used it for 4bit subticks to byte data.