Support use of local link checker

[chalin]

When testing locally, if you have htmltest in your path, that command will be used. Otherwise, a copy will be fetched.

Closes #546

[chalin]

From the build log, we can see that htmltest is still being fetched, and links are being checked:

9:10:16 AM: hugo ...
9:10:16 AM: 	--minify
9:10:19 AM: Building sites …
9:10:19 AM: Total in 3269 ms
9:10:19 AM: curl https://htmltest.wjdp.uk | bash -s -- -b tmp/bin
9:10:19 AM:  ...
9:10:20 AM: wjdp/htmltest info checking GitHub for latest tag
9:10:20 AM: wjdp/htmltest info found version: 0.14.0 for v0.14.0/linux/amd64
9:10:22 AM: wjdp/htmltest info installed tmp/bin/htmltest
9:10:22 AM: tmp/bin/htmltest
9:10:22 AM: htmltest started at 01:10:22 on public
9:10:22 AM: ========================================================================
9:10:22 AM: ✔✔✔ passed in 243.385591ms
9:10:22 AM: tested 397 documents
9:10:22 AM: ​
9:10:22 AM: (build.command completed in 23.2s)