fix #764 - support external links in sidebar

[timaschew]

Credits goes to #1012
This PR is based on top of master and has no changes in the Sidebar.vue file compared to the original.

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:
before sidebar is not rendered at all, because of an error

after sidebar is rendered with external links properly

See demo: https://hungry-yonath-bfd0a1.netlify.com/

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[narration-sd]

This is great, @timaschew & @ulivz, kind thanks for pushing this ahead -- and I'd like to put it right away to good use.

I'm trying variations on this:

    npm install vuejs/vuepress#141bd11 --save-dev

...and in fact that incantation seems to find your work, as it gives back the full commit id, but then fails as:

npm ERR! Can't install github:vuejs/vuepress#141bd11e4b7e1b4d1bee53a49c232b11a36516cd: Missing package name

Digging on that Missing package name error, I ended up here, which is not promising on its own.

How would you propose I pick this up, other than editing it into my node_modules or global source??

[rasapetter]

Author of the original PR here. Thank you for adopting my branch! Great to have this merged.

I started rebasing the branch a couple of days ago when I had a few minutes to spare, but the conflicts looked gnarly enough that I didn't follow through. Glad to see it wasn't as bad as it seemed.