make italic and bold behave more like other editors

[StaticNoiseLog]

An attempt to make italic and bold behave more like other editors. In particular:

• Italic and bold still continue to new lines, but an empty line interrputs it.
• To be valid, the starting markers must be immediately followed by non-whitespace and the ending markers must be preceded immediately by non-whitespace.

See discussion here: https://vi.stackexchange.com/questions/11116/how-to-make-a-syntax-region-stop-on-empty-lines

In addition, the line hi def link markdownCode Delimiter causes in-line code that is framed by backticks to be colored the same as code indented with four spaces.

I have not tried to change markdownBoldItalic and markdownStrike.

[StaticNoiseLog]

Apologies, this does not work well enough. Withdrawn.