fix: Enable flags for port and cmd

[shortdiv]

- Summary
Currently, flags for cmd and port don't work. This PR ensures that flags are added and override the dev block config in the netlify.toml file.

- Test plan
Add the following dev block to your netlify.toml file:

[dev]
  command = "yarn start" 
  port = 3000
  publish = "dist"

Then run the following command: netlify dev --port 3000 or netlify dev --command="yarn start"

- Description for the changelog
This PR joins the option flags with the dev config in the netlify toml and ensures that the former overrides the latter.

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