Support Multiple Replacers

[michaeljpeters]

What do you think about adding a new file that contains the following objects:

replacers.lua

default = {
    dir_to_session_filename = ..., -- <current dir_to_delimited_session_filename>
    session_filename_to_dir = ..., -- <current delimited_session_filename_to_dir>
}

git_repo = {
   -- ... The git repo implementation ...
}

Then, in configuration, you could do something like

... .setup({ replacer = require('session_manager.replacers').git_repo, ... })

This would make it easier to support new replacer implementations.

Originally posted by @michaeljpeters in #64 (comment)