add hashtags extension
bump version
set EXT_HASHTAGS when hashtags is enabled
update hashtag extension
new usernames extension
update peg-markdown

- improved autolinker
- conversion of hashtag links
update peg-markdown: prefix hashtag links with '#'
hashtags: permit underscores
new extension: spoilerblock
updated peg-markdown
updated peg-markdown (autolinker fix)
support full user handles with domain part
update peg-markdown (exclude trailing period from usernames)
write own README
rename fork to parkdown
update version
do not include trailing periods/commas in autolinked URLs
update peg-markdown
Repair MarkdownTest#test_that_extension_methods_are_present_on_markdown_class .