Introduce new --style called default that is not necessarily same as full

[Enselic]

There is a discussion in #1988 whether or not to include header-filesize in the default --style. The safest way forward is to be conservative and not include header-filesize by default. At least not yet. We can maybe make it default later.

To achieve that, we need a new style. I propose we simply call it default.

So we would have:

                     * default: enables recommended components (can change across versions) (default).\n  \
                     * full: enables all available components.\n  \
                     * auto: same as 'default', unless the output is piped.\n  \
                     * [... same as before ...]

Having (can change across versions) in the description is not very elegant. For now I just put it there for clarity. I think the implementation will be quite easy if we can agree on e.g. a name for the new style. We don't necessarily need to call it default.

It would be good to also take the opportunity to get rid of auto while we are at it. See #1743.

[sharkdp]

Sounds good to me. I am in favor of introducing default.