support running many times in loop with one instance

[sherry0429]

fix bug when

for i in range(0, 100):
    toaster.show_toast(...., duration=300)
    time.sleep(1)

and program create window instance many times

and remove destroy window after toast show.
now window will be destroy after this:

toast.destroy()

or

with CustomToastNotifier() as toaster:
    toaster....

text codes:

import time


count = 0
with CustomToastNotifier() as toaster:
    while True:
        time.sleep(3)
        toaster.show_toast("Hello World!!!",
                           "Python is 10 seconds awsm!",
                           icon_path="custom.ico",
                           duration=100)
        count += 1
        if count >= 5:
            break

[sherry0429]

@jithurjacob I add some function here, could you merge this PR ? it's will allow user use this through with block, and run many times in loop without create window manytimes.

thanks.