Key-value attributes are not general enough

[Omikhleia]

The syntax for attribute values in key-value pairs ({ .... key=value } attributes) is not general enough:

lunamark/lunamark/reader/markdown.lua

Lines 155 to 156 in ae3091a

	parsers.attrvalue = (parsers.dquote * C((parsers.alphanumeric + S("._- "))^1) * parsers.dquote)
	+ C((parsers.alphanumeric + S("._-"))^1)

FWIW, Pandoc allows more things:
https://github.com/jgm/pandoc/blob/bc670665a1ce18ddc48531867b7b77c2f3493f49/src/Text/Pandoc/Readers/Markdown.hs#L658-L662

I am not reading Haskell well, but at least:

• single quoted value key='value'
• some escaping things
• wider range of characters (i.e. not restricted as currently to alphanums and some additions).

Found the issue while trying to have a width="15%" attribute pair ;)