Fix hashes in code blocks rendered as hashtags

Previously, our hashtag parser would indiscriminately replace
hashtag-like text with hashtag HTML -- including in places it shouldn't
have, like inside code blocks. Along with the v1.7.0 changes to
writeas/saturday, this fixes that and closes #6.

As a bonus, strings of #spaceless#hashtags#in#a#row are now rendered
correctly.