Add hyperlink to both standard libraries and third-party libraries

[Wolf359Stella]

Refers to issue #825 .
This pull request implements the following:

• Adds a new command line argument, -l / --link-library, which uses the same format as -e / --edit-url.
• Pass link-library to jinja env as link_library
• Adds the function get_stdlib_doc_link to pdoc/render_helpers.py, which will be used to identify if a string is a reference to a standard python library. If so it returns a hyperlink to it, otherwise it returns None
• Use get_stdlib_doc_link along with link_library dictionary as additional test for type annotation in both linkify and link functions from pdoc/render_helpers.py.

The result is that the documentation now adds hyperlink to both standard libraries and third-party libraries.
I run the pytests and some are failing. I was able to group them into 3 groups:

• Failing because standard libraries now have hyperlinks
• enum.txt, which required to update a substring that looked like it was caused just by python version differences.
• demopackage_dir.html, which I based on search pre-processing and I can´t be sure it is ok.
  I fixed all, as asked in the guideline

[Wolf359Stella]

Hi, I have tried several times but was not able to get all testcases passing. The one failing is on Windows, which I dont use. Could someone help me with this?