Fix typo in show_with() error message
Refactor option support

Define the extension options separate from the general options.

Test for the presence of an option before trying to evaluate it. This
protects against options being unset after they were first initialized.
Implement support for Markdown fenced code blocks

Define a setting g:PreviewMarkdownFences. When set to 1, enable support
for fenced code blocks in Markdown files. This may also be specified
per-buffer as b:PreviewMarkdownFences.
Merge branch 'markdown_fences'