Refactor readers and remove MARKUP

Add a `Readers` class which contains a dict of file extensions / `Reader`
instances. This dict can be overwritten with a `READERS` settings, for instance
to avoid processing *.html files:

    READERS = {'html': None}

Or to add a custom reader for the `foo` extension:

    READERS = {'foo': FooReader}

This dict is no storing the Reader classes as it was done before with
`EXTENSIONS`. It stores the instances of the Reader classes to avoid instancing
for each file reading.
Disable asciidoc files for tests
Add a signal to give access to the dict of Reader classes.
Move the "find image with an empty alt" block in a function.
Add documentation for readers.
Merge pull request #1011 from saimn/readers

Refactor readers and remove MARKUP. Fixes #866