Add '--encoding' option

This allows to handle unicode documents properly.
Use ENCODING as name for the --encoding argument
Use encoding='ascii' as default for function calls
Add missing utf8.rst test document
Fix consistency issues with tests for the encoding
Revert "Use encoding='ascii' as default for function calls"

This reverts commit ce25935e0a28c1c9687ca138a0111349554adb87.
Use default encoding of io.open

This makes the behaviour platform specific, but it can be hoped that the
actual encoding used is not of much importance (for most cases).
Release 0.3.0
Adding heading for `lint_file`
Filled out documentation for `lint_file`
Added returns documentation
Relocating returns text
Release 0.3.1
Starting on regression test for headings errors that have `None` line numbers
Filled out test and caught regression
Fixing both `line` and `source`
Added assertion for source as well to prevent regression
Release 0.4.0
Delving into docutils again. Found how many things we have missed. It looks like the new regression is unveiled via transformers
Starting a new flavor of copy/pasta