Line-ending-agnostic diff

[carols10cents]

From https://github.com/azerupi/mdBook/pull/237#issuecomment-294225041:

Ahh, line endings are the issue here.... I made this PR on Windows, so windows stuff succeeds, but linux stuff fails. Hrm.

..... I am not sure how to fix this, given that if I change it the other way, it will fail on Windows. Hrm.

Ideas:

• call lines() on the files and compare over the lines?
• shell out to diff and use --strip-trailing-cr (idk the windows equivalent though)

and this should probably also be an option, not the default?