Appending to key does not work

[AltayAkkus]

Hello,
I've noticed that using .append() on a PickleShare Key does not represent the changes that were made.

Example

Command Line 1:

>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack'] = []
>>> db['stack']
[]

Command Line 2:

>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack']
[]

Command Line 1:

>>> db['stack'].append("Hello World!")
>>> db['stack']
['Hello World!']

Command Line 2:

>>> db['stack']
[]

In fact, I added my own debugging, and the .append() does not even trigger the __setitem__ function of the PickleShareDB class.

>>> from pickleshare import *
>>> db = PickleShareDB('test')
This is the debugged version.
>>> db['testvalue'] = 1234567
Set item called with key:testvalue and value:1234567
>>> db['stack'] = []
Set item called with key:stack and value:[]
>>> db['stack'].append("Hello")

Fix

Since the __setitem__ magic method only triggers when the value of a key is set, or updated, it does not trigger when you .append() something.
Also, there is no magic method that triggers when an .append() happens
So I suggest adding an .append() function:

    def append(self, key, value):
        currentValue = self[key]
        currentValue.append(value)
        updatedValue = currentValue
        self[key] = updatedValue

Test

Command Line 1:

>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack'] = []
>>> db['stack']
[]

Command Line 2:

>>> db = PickleShareDB('C:\\Users\\NULL\\AppData\\Local\\Temp\\test')
>>> db['stack']
[]

Command Line 1:

>>> db.append('stack', 'Hello World!')
>>> db['stack']
['Hello World!']

Command Line 2:

>>> db['stack']
['Hello World!']

It would be nice if you could overwrite the default .append() function, like db['stack'].append(0) but since every db['key'] is not an instance of the class PickleShareDB but a list, I don't want to mess with it :D