Fixed "--config" command-line flag and added documentation.
Added new plugin.

The plugin will generate user defined formatted modification time of page in
which defined variable is used.
Added new plugin.

The plugin will generate correct path to files from any pages which is given
by the user in an absolute way (leading / is not mandatory though). It simply
adds a relative path to web-root in front of the value given.
Fixed input and output directory manipulation.

If one forget to add a trailing slash in the input or output global variables
then filenames were build incorrectly. The slash between the input (output)
part and the rest of the page source file was forgotten resulting in an
inexistent file path.

I add some code to ensure that both variables (input and output) embed a
unique trailing slash when initializing the whole process (in Site::init).
Merge pull request #7 from b3/master

Fixing the --config command-line option.