Make .standard_todo.yml narrow to migrate smoother

[mtsmfm]

Currently, --generate-todo generates todo.yml which ignores all rules.

$ cat foo.rb
foo = 'foo'
$ standard --generate-todo
$ cat .standard_todo.yml
# Auto generated files with errors to ignore.
# Remove from this list as you refactor files.
---
ignore:
- foo.rb:

This patch makes todo.yml narrow.
It prevents increasing todos.

$ cat foo.rb
foo = 'foo'
$ standard --generate-todo
$ cat .standard_todo.yml
# Auto generated files with errors to ignore.
# Remove from this list as you refactor files.
---
ignore:
- foo.rb:
  - Lint/UselessAssignment
  - Style/StringLiterals

[searls]

Sorry it took me so long to review this! I wanted to pull down the code myself to test it.

[searls]

Landed in 1.1.7