Show path to plugins directories in `bzr version` output

[Alexander Belchenko]

This is inspired by http://stackoverflow.com/questions/11454372/how-to-install-a-bzr-plugin, especially this answer http://stackoverflow.com/a/11454397/65736

I don't think that using python and doing magic witrh bzrlib is good UX.

`bzr version` already shows a lot of useful information. Maybe it will be good to have there one more bit: list of plugins directories, where bzr will search for plugins. Today it's not very obvious, especially if you have BZR_PLUGIN_PATH and you forget about it, and because built-in plugins could be installed along bzrlib or in Program Files if you have installed bzr.exe.