display which theme is the default one

[sblondon]

The user knows which theme is used by default with --list-themes parameter.

This is a partial screenshot of the output:

Only the config.colored_output displays the (default) string. I didn't add it in the other case because it's really minimal but I will add it if you want.

[sblondon]

@Enselic I added an integration test.

[sblondon]

@Enselic @keith-hall I rebased and fixed the conflicts occurred since the first commits. Do you think about blocking elements preventing the merge of this PR?