Better default for cell delimiters

[jonathan-laurent]

Some style guides (such as YASGuide) recommend using the following section headers:

#####
##### Section Name
#####

This conflicts with the default cell delimiters regexps of Juno, which would create three cells when seeing the header above.

Proposed fix: replace the ## regexp by ##\s, as suggested by @pfitzseb on Slack.

Signed-off-by: Jonathan Laurent jonathan.laurent@cs.cmu.edu

[aviatesk]

thanks @jonathan-laurent !