Telegram-export

Fixes:
Don't get entities of blacklisted IDs (fix 84)
Fix guess_extension exception on None inputs for mime types (88)
2FA password config field comment clarity improvement (89)

Incremental update - update to newest Telethon and add a 2FA password code field to the config.

A lot of nice little tweaks, and also [brand new proxy support](https://github.com/expectocode/telegram-export/commit/1e13a2d7a82c1a8d01e4820cdae6040408a8f685) Thanks to xkord for making this one happen. If there's something you wish the program did, consider making a PR. We appreciate them a lot.

You can check out the new proxy config syntax by looking in [config.ini.example](https://github.com/expectocode/telegram-export/blob/master/config.ini.example) or running `telegram-export --help`

Other changes:

- There was a column created with the [wrong datatype](https://github.com/expectocode/telegram-export/commit/a4ce8d94bb090a0f236f705d2e7ca235bb5bc0bb), which is not a breaking change but you may want to alter your tables if you're a perfectionist.
- The program now only mentions "not admin" [when it's relevant](https://github.com/expectocode/telegram-export/commit/eec74b3f84889a67bd02c293a988ea3a807ef5bd).
- A missing entity for some URLs [is now dumped](https://github.com/expectocode/telegram-export/commit/2c88a23fa7eb7cfa763a53b73785811fa0412d19).
- Config and help explanations have been improved to make things clearer and easier. If you find some other annoyances, let us know and we'll get to it (at some point).
- Some other [bug fixes](https://github.com/expectocode/telegram-export/compare/95261ec4f1409e9e4464f43ec45ba6f43e366b52...d5c33cea8a2c4ff5276dac43c30e2dfc00fbff15).

**Basic functionality is now fully implemented** - messages, users, channels, supergroups, participants, and admin log.

Next on the list is a nicer cli interface and somewhere in the future could be a GUI. We also want to reach a stable point with the DB schema and get rid of bugs we don't even know about yet :P