Add option to add external links to source code

[twiss]

Q	A
Bug fix?	no
New feature?	yes
Breaking change?	no
Deprecations?	no
Tests added?	no
Fixed issues
License	Apache-2.0

This feature is meant as a compromise between copying all source code to the documentation directory (outputSourceFiles = true), and having no links to the source code at all (outputSourceFiles = false). Instead, this option allows you to link to an external source code repository (e.g. GitHub or GitLab).

• The templates.default.externalSourceLinks.urlPrefix option points to the location where the source code is hosted; the path of the file is appended to it.
• The templates.default.externalSourceLinks.linenoPrefix option defaults to 'L' and refers to the L in #L5 which points to line 5 on both GitHub and GitLab.

This was manually tested against v3, not v4, because I couldn't get npm installing v4 from GitHub or a sibling directory to work. I also didn't see any tests for the outputSourceFiles feature, so I wasn't sure how to test this. All existing tests pass in v4 too, though, and the patch applied cleanly, so I imagine it should work.

Here is another example for how I intend to use this, automatically taking the repository url and version tag from package.json: https://github.com/twiss/openpgpjs/blob/v5-test/.jsdocrc.js. This should be convenient when updating the documentation whenever you release a new version, for example. That should also minimize the risk that the linked source code doesn't match the processed source code.