Create a method that can find channels (either by query or external sources)

[pushshift]

For the channels.py script, add method(s) that will enable discovery of new channels. Ideally, we would have the capability of discovering all publicly available channels. Also, being able to discover new channels as they are added to Telegram would be extremely beneficial.

[zsavvas]

Added support to search for channels using external sources (Telegago and Lyzem search engines) in commits 17cf6c2 and a4f1a69.
See methods search_channels_lyzem and search_channels_telegago in channels.py
Things to keep in mind: Telegago has more results as it is a google search engine for Telegram. However, it is slower as we need to render Javascript and because each page has a limited number of results we need to fetch more pages. On the other hand, Lyzem has fewer results but it is faster (no need to render Javascript) and we can fetch up to 100 results per request (i believe that most of the queries will have less than 100 results)