* Add check subcommand

* Add some brief documentation for the check subcommand

* Start working on parallel link checks

* Check all external links in Site

* Return *all* dead links in site

Closes #674

To ensure we will get the right `content`
directory.
Fix #629

Closes #607

Only happens for sections with lang != default

trim_{start, end} is introduced in rust 1.30.0 and
trim_{left, right} is deprecated since 1.33.0.

Closes #576

Closes #537

Closes #521

Closes #533

Closes #385

To override all child pages

Closes #397
Tests from PR #434

Closes #413
Closes #408

Fix #396

Fix #495

Closes #485

Close #446
Close #260
Close #478
Close #284
Close #480

Fix #122

Fix #205