Accept `mdown` file extension for Markdown files.

This extension is sometimes used for Markdown files and is the default
for a few editors, such as Sublime Text.
force argparse output to be unicode in Py2
add feedback to user and refactor 'autoreload' code
Fix outdated link to documentation
Merge pull request #856 from avaris/non-ascii-path

force argparse output to be unicode in Py2
bugfix and tests for no settings path (no -s option)
Merge pull request #857 from avaris/feedback

add feedback to user and refactor 'autoreload' code
Merge pull request #855 from lilix/accept-mdown-file-extension

Accept `mdown` file extension for Markdown files.
Improve wording of "cannot find" errors
Add content_object_init to list of signals