Issue 12765 - dmd needs a "--version" option like gdc

- move printing of compiler logo into separate function

- add --version switch to only print version and copyright

[Issue 12765](https://issues.dlang.org/show_bug.cgi?id=12765 "dmd needs a "--version" option like gdc")
Not quite, but I dislike mixed styles in a single file (see [above](https://github.com/MartinNowak/dmd/blob/cf73ebbff34311f0833c53a0391244dc53fd8390/src/mars.c#L914)).