Ensure npm install runs once at a time

[zakj]

On starting up the Lektor server, the server_spawn and before_build_all events both fire, each one spawning npm install. This ensures that only one runs at a time.

[codecov-io]

Current coverage is 97.82% (diff: 100%)

Merging #9 into master will increase coverage by 0.26%

@@             master         #9   diff @@
==========================================
  Files             1          1          
  Lines            41         46     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits             40         45     +5   
  Misses            1          1          
  Partials          0          0          

Powered by Codecov. Last update 677c839...f33edf4

[zakj]

Ha, you caught me trying to avoid threaded tests. ;) I'm glad you did, though—I think I've arrived at a slightly better implementation while making it testable.

Let me know if there's anything else I can fix up, and thanks so much for this plugin (and your work on Lektor itself)!