Chat post message, ephemeral, and me commands

[netromdk]

Fixes #80 and #82.

• Fixed arguments parsing so it supports string inputs, like -arg 42 "hello world", for instance
• Doesn't reparse preparsed arguments from sys.argv for single command arguments
• Implemented chat.postmessage command:

usage: chat.postmessage [-h] [-c CHANNEL] [--as-user] [-u USER]
                        [--no-markdown]
                        text

Post message to a channel on Slack.

positional arguments:
  text                  Text to post.

optional arguments:
  -h, --help            show this help message and exit
  -c CHANNEL, --channel CHANNEL
                        Channel ID to post to.
  --as-user             Post as authed user instead of as bot.
  -u USER, --user USER  Bot user name to use. Must be specified when not using
                        --as-user.
  --no-markdown         Disable Slack markup parsing.

• Implemented chat.postephemeral command:

usage: chat.postephemeral [-h] [-c CHANNEL] [-u USER] [--as-user] text

Post ephemeral message to a channel on Slack that is only visible to assigned
user.

positional arguments:
  text                  Text to post.

optional arguments:
  -h, --help            show this help message and exit
  -c CHANNEL, --channel CHANNEL
                        Channel ID to post to.
  -u USER, --user USER  User ID to post ephemeral message to.
  --as-user             Post as authed user instead of as bot.

• Implemented chat.memessage command:

usage: chat.memessage [-h] [-c CHANNEL] text

Post a me message to a channel on Slack.

positional arguments:
  text                  Text to post.

optional arguments:
  -h, --help            show this help message and exit
  -c CHANNEL, --channel CHANNEL
                        Channel ID to post to.

[netromdk]

I will also add the chat.postephemeral command here.