formatter: add pad_line_comments option

[badouralix]

Fixes #104 using braydonk/yaml#16

Add an option to change the number of padding spaces between the yaml content and the first # of a line comment
Keeping 1 as the default value for backward compatibility, although using 2 would make sense as it would be compliant with yamllint default rules

Example of usage :

$ cat comments.yaml
key: "value" # line comment

$ yamlfmt --formatter pad_line_comments=2 comments.yaml

$ cat comments.yaml
key: "value"  # line comment

Also remove unneeded entries from the go sum file

[braydonk]

LGTM