Support histsize & erasedups (closes #29), pure php implementation of libedit::readline_list_history

Well everything is in the title...

First we can now get rid of the `Psy\Readline\LibeditTransient` class, since listHistory is now fully supported, thanks to the pure php implementation of libbsd's `strunvis` in `Psy\Util\String::unvis()`.

There are also two new rc settings:
  * eraseDuplicates: if set to `true`, the history behavior is the same as bash with `HISTCONTROL=erasedups`
  * historySize: controls the maximum number of entries the history can contain. If set to zero (or null or anything < 1), allow an infinite number of entries. (We propably should provide a sensible default here, ie 1024 since the history can quickly grow under intensive use, especially with `eraseDuplicates=false`).