Add DNS provider for BookMyName

[ldez]

• adds a description to your PR
• have a homogeneous design with the other providers
• add tests (units)
• add tests ("live")
• add a provider descriptor
• generate CLI help, documentation, and readme.
• be able to do: (and put the output of this command to a comment)

  make build
  rm -rf .lego
  
  BOOKMYNAME_USERNAME=xxx \
  BOOKMYNAME_PASSWORD="yyy" \
  ./dist/lego -m [email protected] --dns bookmyname -d *.example.com -d example.com -s https://acme-staging-v02.api.letsencrypt.org/directory run

  Note the wildcard domain is important.
• pass the linter
• do go mod tidy

Ping @euskadi31, can you run the command (with your domain, email, credentials, etc.)?

How to test this PR?

1. You need Go
2. Checkout the PR:

   git clone https://github.com/ldez/lego.git
   cd lego
   git checkout feat/bookmyname

3. Compile lego:
   • if you have make: make build
   • if you don't make make: go build -o dist/lego ./cmd/lego
4. Run the following command with your own information (email, domain, credentials):

   BOOKMYNAME_USERNAME=xxx \
   BOOKMYNAME_PASSWORD="yyy" \
   ./dist/lego -m [email protected] --dns bookmyname -d *.example.com -d example.com -s https://acme-staging-v02.api.letsencrypt.org/directory run

   The wildcard domain is important
5. Before each run of the command, you should clean your local environment:

   rm -rf .lego

Closes #1013