Do not call watchConfig() when not in watch mode

[anthonyfok]

See #1772

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@spf13, does this look okay to you? :-)

[anthonyfok]

(Note to self: Next time, test before pushing a commit for the first time. 😉)

Finally got my PR to work as expected. My initial push erroneously used viper.GetBool("watch") which was never set and thus simply disabled watchConfig() indiscriminately... Oops!

I then discovered that the config watch only worked the first time when I do echo "" >> docs/config.toml, and then it got stuck, hence I added one more line to fix the problem. From the commit log:

Also, force DisableLiveReload to true when running "hugo --watch" (build-only non-server mode) to prevent livereload.ForceRefresh(), which would end up blocking watchConfig() forever, from being called because livereload.Initialize() is never called in this case.

This fixes the bug where "hugo --watch" could only reload config.toml once before it gets stuck for good at livereload.ForceRefresh().

This is also consistent with Hugo's existing behaviour: Non-server "hugo --watch" has never injected livereload.js since the inception of the "watch" feature in Hugo v0.12.

[spf13]

This looks great. You've fixed two bugs in one commit. Merged as e8eb618

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