Add the $is_syncing check to hook_install / installed

[andybroomfield]

Fix #17

Alternative fix to #18. Better to not modify site config or install extra fields during the config import operation.

BHCC are running into the issue mentioned in #17, and as mentioned on the PR

18 it may actully be prefferable not to respond to any hook_install / hook_installed events during a config sync operation.