Add entryPoint option

[jhuleatt]

Fixes #16

Adds an entryPoint option to allow for firebase.json files that aren't in the repository's root.

The name entryPoint was chosen to feel similar to a workflow file's entrypoint, which doesn't work with JS Actions, but also maintain camel case to match the other options for this Action.

Example:

action-hosting-deploy/.github/workflows/deploy-preview.yml

Lines 26 to 31 in 350fcd4

	with:
	repoToken: "${{ secrets.GITHUB_TOKEN }}"
	firebaseServiceAccount: "${{ secrets.FIREBASE_SERVICE_ACCOUNT }}"
	expires: 30d
	projectId: action-hosting-deploy-demo
	entryPoint: "./demo"

[github-actions]

Deploy preview for 846da00:

https://action-hosting-deploy-demo--pr16-add-entrypoint-optio-daecu2og.web.app

_{(expires Wed, 16 Sep 2020 23:32:54 GMT)}

_{firebase-hosting-preview-action}

[davideast]

LGTM.