Options through meta tags only works while generating from string #254

Hi,

I was trying to set options using meta tags while generating a pdf from an html file, but they were not used.
While looking at pdfkit's code I noticed that the meta tag parsing is only enabled when using from_string

The documentation doesn't seems to mention this restriction.
Is there any reason to only allow meta tags options for string input ?

An easy workaround would be to read the html file and use from_string instead of from_file but it just seems weird.
Especially since _find_options_in_meta uses both strings and file-like objects