fix missing tab in Makefile template
Merge pull request #1511 from olivierverdier/makefile_tab

Fix missing tab in Makefile template
Make StaticGenerator skip content sources. Refs #1019.

This change partially addresses issue #1019, by teaching Pelican to distinguish
between static files and content source files. A user can now safely add the
same directory to both STATIC_PATHS and PAGE_PATHS (or ARTICLE_PATHS). Pelican
will then process the content source files in that directory normally, and
treat the remaining files as static, without copying the raw content source
files to the output directory. (The OUTPUT_SOURCES setting still works.)

In other words, images and markdown/reST files can now safely live together.

To keep those files together in the generated site, STATIC_SAVE_AS and
PAGE_SAVE_AS (or ARTICLE_SAVE_AS) should point to the same output directory.

There are two new configuration settings:

STATIC_EXCLUDES=[]  # This works just like PAGE_EXCLUDES and ARTICLE_EXCLUDES.
STATIC_EXCLUDE_SOURCES=True  # Set this to False to get the old behavior.

Two small but noteworthy internal changes:

StaticGenerator now runs after all the other generators. This allows it to see
which files are meant to be processed by other generators, and avoid them.

Generators now include files that they fail to process (e.g. those with missing
mandatory metadata) along with all the other paths in context['filenames'].
This allows such files to be excluded from StaticGenerator's file list, so they
won't end up accidentally published. Since these files have no Content object,
their value in context['filenames'] is None. The code that uses that dict has
been updated accordingly.
Merge pull request #1509 from foresto/static-files-anywhere

Make StaticGenerator skip content sources. Refs #1019.
Ignore empty metadata. Fixes #1469. Fixes #1398.

Some metadata values cause problems when empty.  For example, a markdown file
containing a Slug: line with no additional text causing Pelican to produce a
file named ".html" instead of generating a proper file name.  Others, like
those created by a PATH_METADATA regex, must be preserved even if empty,
so things like PAGE_URL="filename{customvalue}.html" will always work.
Essentially, we want to discard empty metadata that we know will be useless
or problematic.  This is better than raising an exception because (a) it
allows users to deliberately keep empty metadata in their source files for
filling in later, and (b) users shouldn't be forced to fix empty metadata
created by blog migration tools (see #1398).

The metadata processors are the ideal place to do this, because they know
the type of data they are handling and whether an empty value is wanted.
Unfortunately, they can't discard items, and neither can process_metadata(),
because their return values are always saved by calling code.  We can't
safely change the calling code, because some of it lives in custom reader
classes out in the field, and we don't want to break those working systems.
Discarding empty values at the time of use isn't good enough, because that
still allows useless empty values in a source file to override configured
defaults.

My solution:
- When processing a list of values, a metadata processor will omit any
  unwanted empty ones from the list it returns.
- When processing an entirely unwanted value, it will return something easily
  identifiable that will pass through the reader code.
- When collecting the processed metadata, read_file() will filter out items
  identified as unwanted.

These metadata are affected by this change:
author, authors, category, slug, status, tags.

I also removed a bit of now-superfluous code from generators.py that was
discarding empty authors at the time of use.