display which theme is the default one in basic output

[sblondon]

The previous PR #2838 shows which theme is the default one in colored output when --list-themes argument is used. This PR does the same with not colored output (as suggested by @Enselic ).

The output is:

I think about another commit to refactor the internal of list_themes() function (so it should not the squashed with the previous one).

Current code:

    if config.colored_output {
        for theme in assets.themes() {
            // display colored theme
            // display "Further themes can be installed..."
    } else {
        for theme in assets.themes() {
           // display basic theme
        }
    }

To:

    for theme in assets.themes() {
        if config.colored_output {
            // display colored theme
        } else {
           // display basic theme
       }
    }
    if config.colored_output {
            // display "Further themes can be installed..."
    } 

What do you think about it?

[sblondon]

I added a new commit with the refactoring explained in the previous comment. So you can review it directly.

IMO, it's better with this refactoring.
However if you don't like it, I will drop the commit.

[Enselic]

Looks reasonable! I had one nitpick but we can fix that later, to avoid another round of back and forth.

Thanks!