[Feature Request] Set title attribute in html

[SomervilleTom]

Add a directive or setting that allows markdown-pdf to set the title attribute of a generated file to be an arbitrary string.

If the filename of the source file is 'index.md', the current behavior is to generate an html file with the following title attribute:

<title>index.md</title>

This means that after each change to the source file, the resulting .html file must be edited by hand to change the title attribute.

If the desired title is "Some Interesting Page", make it possible for the generated file to emit the following title attribute:

<title>Some Interesting Page</title>