If a plugin's target (constructor or function) doesn't satisfy specific
parameters, it'll never be called. This documents what the parameters must
contain to be run at all.
Related to: #159
It updates the documentation to separate which parameters are static and
which are changed on each line. Using the latter parameters on plugins which
are only run once per file isn't very sensible.
The documentation for the `FileProcessor` class used `indect_char` while the
class itself uses the more sensible name `indent_char`. This updates both the
docstring as well as the documentation.
