Add configurable prompt

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First pass at allowing the repl prompt to be configured using a template string. The prompt configuration is set in the config file with the default template string ${ro}${w}>.

Currently the prompt supports the following:

• ${ro} read-only mode. When Slacker is running in read-only mode this will be replaced with the contents of read_only_str config variable (default = (read-only))
• ${w} current workspace Slacker is using.

Additional entities could be added:

• {u} for username, "username"
• {uid} for user ID, "U8C3V3OQ"
• {wid} for workspace ID, like "T9F3H7VLP"
• {url} for workspace URL, like "https://workspace.slack.com/"

Other identifiers could easily be supported using this method, as discussed in #76.

This is a pretty trivial implementation as it doesn't have any lexical analysis, strictly a string replace. If we want to support more complex logic we'd have to subclass the Template class with our specific logic, or roll our own entirely.

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Added comments.

Yeah I was going to write a wiki entry for this once we agreed on the template identifiers and general implementation.