Add the ability to use the domains API

[chengfang-blutui]

This PR adds the ability to use the Blutui Agency Domains API:

// Get all domains
blutui.agency('myAgency').domains.list()

// Retrieve a domain
blutui.agency('myAgency').domains.get('DOMAIN_ID')

// Create a new domain
blutui.agency('myAgency').domains.create({ name: 'example.com', project: 'PROJECT_ID'})

// Update project of a domain
blutui.agency('myAgency').domains.update({ project: 'PROJECT_ID'})

// Remove a domain
blutui.agency('myAgency').domains.remove('DOMAIN_ID')

// Refresh token of a domain
blutui.agency('myAgency').domains.refresh('DOMAIN_ID')

// Verify DNS settings of a domain
blutui.agency('myAgency').domains.verify('DOMAIN_ID')

// Search domains of your agency
blutui.agency('myAgency').domains.search('{ name: 'example'})