feat: add skipOn and onlyOn with headed or headless mode (cypress-io#39)

* add skipOn headed or headless

* add onlyOn and tests

fix: normalize browser name comparison

feat: use new Cypress.isBrowser method if available

feat: add environment variable support (cypress-io#12)

```js
// run tests with 
// CYPRESS_ENVIRONMENT=staging npx cypress run
import {onlyOn} from '@cypress/skip-test'
const stubServer = () => {
  cy.server()
  cy.route('/api/me', 'fx:me.json')
  cy.route('/api/permissions', 'fx:permissions.json')
  // Lots of fixtures ...
}
it('works', () => {
  onlyOn('staging', stubServer)
  ...
})
```

* chore: add prettier

* add .vscode settings

* feat: add ENVIRONMENT support

feat: add `isOn` function

You can check the condition against a browser name or an environment
yourself.

```js
import {isOn} from '@cypress/skip-test'
it('loads users', () => {
  // when running on Windows locally, the backend is not running
  // thus we need to stub XHR requests
  if (isOn('windows') && isOn('localhost')) {
    cy.server()
    cy.route('/users', 'fixture:users')
  }
  cy.visit('/')
  cy.get('.user').should('have.length', 10)
})
```

chore: add link to filtering PR, close cypress-io#7

feat: split functions from support file

BREAKING CHANGE: if you want to use custom commands, you need
to explicitly require the support file like this.

```js
require('@cypress/skip-test/support')
```

This way you can import individual functions without adding them
as custom commands.

fix: normal module export instead of es6

feat: callback form, close cypress-io#6

feat: export functions, close cypress-io#5