Refactor logging handling

Old system was using manual string formatting for log messages.
This caused issues with common operations like exception logging
because often they need to be handled differently for Py2/Py3
compatibility. In order to unify the effort:

 - All logging is changed to `logging.level(msg, arg1, arg2)` style.
 - A `SafeLogger` is implemented to auto-decode exceptions properly
in the args (ref #1403).
 - Custom formatters were overriding useful logging functionality
like traceback outputing (ref #1402). They are refactored to be
more transparent. Traceback information is provided in `--debug`
mode for `read_file` errors in generators.
 - Formatters will now auto-format multiline log messages in order
to make them look related. Similarly, traceback will be formatted in
the same fashion.
 - `pelican.log.LimitFilter` was (ab)using logging message which
would result in awkward syntax for argumented logging style. This
functionality is moved to `extra` keyword argument.
 - Levels for errors that would result skipping a file (`read_file`)
changed from `warning` to `error` in order to make them stand out
among other logs.
 - Small consistency changes to log messages (i.e. changing all
to start with an uppercase letter) and quality-of-life improvements
(some log messages were dumping raw object information).
Corrected typo to improve readability
Allows Typogrify to ignore user specified tags. Refs #1407

Typogrify interferes with certain sections of the output that it should not touch (see #1407 for more details).
This feature adds a setting called TYPOGRIFY_IGNORE_LIST which is a list of tag for Typogrify to ignore.

The following was updated:

 1. readers.py - if TYPOGRIFY_IGNORE_TAGS is present, then use it
 2. settings.ps - default TYPOGRIFY_IGNORE_TAGS to []
 3. contents/article_with_code_block.rst - an article with a code block for typogrify to ignore
 4. updated tests
 5. updated documentation
Merge pull request #1418 from willingc/fix-doc-typo

Corrected typo to improve readability
Merge pull request #1412 from avaris/refactor_logging

Refactor logging handling
Adds reference to SLUGIFY_SOURCE

When working on my site, I wanted SLUGIFY_SOURCE, but I came across PATH_METADATA first and enabled that for a while. Better to include the cross-reference where people can find it.
Merge pull request #1411 from barrysteyn/typogrify-ignore-list

Allow Typogrify to ignore user specified tags. Refs #1407
Fixes #1420: Handle multiple definitions of standard metadata for Markdown
Merge pull request #1445 from avaris/fix_1420

Fixes #1420: Handle multiple definitions of standard metadata for Markdown
Merge pull request #1438 from mlissner/patch-2

Add reference to SLUGIFY_SOURCE