implement a strip_pre configuration option (#218)

[chrispy-snps]

Fixes #218.

Implements a new strip_pre configuration option that provides control over how (and if) leading/trailing blank lines are stripped from <pre> tags.

The default is STRIP to strip all leading/trailing blank lines, which represents a change in behavior. (Previous releases did not perform any blank-line removal in <pre> tags.)

[chrispy-snps]

@AlexVonB, what are your thoughts on the following:

• Do you have a preference for the for the single-line value (STRIP_ONE, STRIP1, STRIP_1, etc.)?
• Are you agreeable to a default value of STRIP (remove all leading/trailing blank lines)?
  • This would be a change in behavior, but a good one I think. @stevecj also mentioned something about this here.

[AlexVonB]

Looks good! I personally feel that the number 1 looks off, so I am in favor for ONE (as well in the code). The default could also be STRIP_ONE, like described in MDN: they ignore only the first newline. But both ist fine with me!

[chrispy-snps]

@AlexVonB - I was thinking of default to STRIP (all) because HTML is often formatted badly, and part of the purpose of Markdown is to be human-friendly and readable.

@matthewwithanm - want to be a tie-breaking vote? :)

[AlexVonB]

Sorry for the late response: let's go with your proposal!

[chrispy-snps]

Thanks @AlexVonB - it's merged!