Add Accept-Language in --create-template

[Dfte]

Hey,

So i have been working with the --create-template option. So first of all it is bugged as of now. I already mentioned the solution here #172 (comment)

The solution would be to install a mono thread version of pywebcopy that you can find here https://github.com/davidwgrossman/pywebcopy.

While using the cloner i realized that cloning some websites (for example twitter) without sending the language-accept HTTP header would result in pywebcopy to clone all pages for all languages and possibly different contents (images, css whatever) which would take a loooooooooooooooooong time.

Thus i think it would be a good idea to add an option that would allow us to pass custom headers ie : --headers "Accept-Language: fr" to retrieve only the french version of the website.

I post this issue as a feature request but I'll take care of it and PR as soon as possible :)