Code review comment for ubuntu-security-tools:json-output

I want to deduplicate arguments, but I do not see an elegant way to do so.

In my previous commit, I attempted ~`json_flags` but abandoned that approach. In addition to adding arguments, `json_flags` would need to remove arguments from StaticAnalysisTool.cmd which requires logic about each tool--unless we also add ~`cmd_flags`. Using a ~`json_flags` approach would need something like:
```
+ cmd_base=["scc", "--exclude-dir", ".git,.hg,.svn,.pc"],
+ cmd_txt=["--no-cocomo", "--ci"],
+ cmd_json=["--format", "json"],
+ cmd_end=["."],
```
(Hopefully no other positional arguments come along!)