Marker for fast default format altering (-f ui option)

New config option `format_edge_marker` gives a string, which should be on the edge of `-f FORMAT` string, to turn into default/configured format. When format string ends with edge marker (e.g. `-f '$energy |'`) effective format becomes `$energy $artist - $album - $title` (when user didn't alter default item format). Prepending format with marker (e.g. ` -f '| $energy'`) gives `$artist - $album - $title $energy`.

My rationale is to avoid regular editing of `list_format_items` option in config file.

This feature can be implemented via plugin, but template function requires at least 4 symbols (`%E{}`) which is many times slower to type and a long way uglier. It would be great to find one symbol for this (right now it's `|`).

Implementing this feature also introduces `print_objs(...)` method, which is like `print_obj(...)` but take collection instead of single object. It gives ability to remove some `beets.util.functemplate.Template` dependencies.