Fixes ptpython/ptipython config and history (#278)

[notdaniel]

This fixes the ptpython/ptipython configuration issue from #278. It will now iterate through a list of all possible config directory locations in order of priority and use the first one it finds, if one exists.

Additionally, very similar logic is now applied to the history filename location to address these 2 bugs:

1. On linux, ptpython no longer keeps the history file inside the config directory, but in a different place entirely. Fixing the config directory issue does not make history work properly.

2. Previously, a history_filename was being passed to embed whether it existed or not. If it doesn't exist, ptpython will try to create it and write to it, but unfortunately it only checks whether the file exists, and not whether its parent directories do. Giving it a history_filename that doesn't exist is fine as long as its parent directory does, but if not, it throws an exception.

It now looks for the history directory in the correct places, and if it cannot find one, it passes history_filename=None to embed.

[sloria]

thanks for the PR!

just a small refactor suggestion. otherwise good to go after that

[sloria]

thank you!