Add breaks extra with ability to hard break on backslashes (issue #525)

[Crozzers]

This PR closes #525 by adding the ability to insert hard breaks when lines end in a backslash.

The feature

By passing extras={'breaks': {'on_backslash': True}}, users can now use backslashes to insert hard breaks, an example of which can be found in the break_on_backslash test case.

Implementation

This has been implemented via a new breaks extra, which contains this new functionality as well as the old break-on-newline functionality. The old extra name has been kept as an alias, so the two following examples are equivalent:

markdown2.convert(text, extras=['break-on-newline'])

markdown2.convert(text, extras={'breaks': {'on_newline': True}})
markdown2.convert(text, extras={'breaks': {'on_newline': True, 'on_backslash': False}})

This was done because I felt that having separate break-on-* extras was unnecessary and that grouping these similar extras made sense.

[nicholasserra]

Looks great, thank you!

[m417z]

Thank you. I assume it should be mentioned here at some point, right? Especially now that a version with this feature was released.
https://github.com/trentm/python-markdown2/wiki/Extras

[Crozzers]

Good shout. I've added a quick page for it here: https://github.com/trentm/python-markdown2/wiki/breaks

[m417z]

Looks good. I think it makes sense to link it from the Extras page I mentioned.

[Crozzers]

Updated the list of implemented extras as well

[m417z]

Looks like the text in the wiki contradicts the actual behavior:

<p>This line will have a line break<br />
But not this one<br />
Because the backslash has to have a space before it</p>

A space before a backslash isn't required.

Also, I created a PR with some improvements: #544

[Crozzers]

Looks like the text in the wiki contradicts the actual behavior

Thanks for spotting this. Updated the wiki to reflect actual behaviour.

Also, I created a PR with some improvements

Very cool! Thank you