Do not call watchConfig() when not in watch mode

Do not call watchConfig() when not in watch mode

See #1772

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.