Make defining npm scripts other than start possible in dev command

The behavior was already described in #119. When using npm and defining configuring another command than npm start in the command field the script execution fails. This is because currently the command get's inserted an additional run argument when npm was detected.

Exception:

Waiting for localhost:7894.npm ERR! missing script: run

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/sjudis/.npm/_logs/2019-05-03T19_11_15_952Z-debug.log

FAIL: 1

I believe that it'd be better to always pass the arguments through to execution because this is what users are used to from the build section of the netlify.toml.

- Test plan

To test this I added a netlify.toml file to this repo as follows.

# example netlify.toml
[dev]
  command = "npm run version"
  port = 7894

Running $ ./bin/run dev leads then to the exception because it's running npm run run version.

🐧  ➡ ./bin/run dev                                                                   [21:10:54]
◈ Netlify Dev ◈
◈ Starting Netlify Dev with undefined
Waiting for localhost:7894.npm ERR! missing script: run

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/sjudis/.npm/_logs/2019-05-03T19_11_15_952Z-debug.log

FAIL: 1

After my changes, the command succeeds.

- Description for the changelog

• Fix npm scripts in dev command

- A picture of a cute animal (not mandatory but encouraged)