Accept multiple config files from command line

Parse config as if it can contain multiple references to config files.
This allows you to pass in multiple config files from the command line

Helps with issues #514 and #703
Upgrade --config to accept an array of files
do not override custom excerpt set in YAML frontmatter

Fixes #933.
Add tests for using custom excerpt
fix indentation of test
rework excerpt to be an accessor method

Instead of setting self.excerpt, make it a method
that returns either the custom excerpt or the pre-
viously extracted excerpt.
remove useless attr_reader for excerpt (is a method now)
Fixing tests, and making it more ruby friendly
new test: ensure the correct excerpt is exposed to liquid

Also shorten test code a little bit.
do not allow markdown etc. in excerpt

Adjust the tests accordingly. Also
add a message for one of the tests.
one more indentation fix
Merge pull request #945 from voidfiles/master

Accept multiple config files from command line
Update history to reflect merge of #945
Merge pull request #946 from maul-esel/preserve_excerpt

Preserve 'excerpt` in YAML Front-Matter
Update history to reflect merge of #946.
expose file path relative to source dir to liquid

Fixes #633.
fix path for posts: must include "_posts"
add cucumber tests for page.path variable
Remove code duplication
Add TomDoc for Jekyll::Command.process_site