Ensure targetFilename is not a boolean before writing

[jasonvasquez]

Changes

• Ensures targetFilename is set to a non-boolean value before checking if it is a markdown file
• Simplifies creation of the options object(s) for the HTML and Markdown output functions

Fixes #58

[matthijsgroen]

Thanks! I need to do a proper dust-off of the project first (replace travis CI) then I'll merge this in :-) It will take some time though...