Add File-based user preferences and scan configuration

[pombredanne]

When I run a scan I should be able to point to a a configuration file that would contain defaults for a scan as well as things such as in these tickets:

• configurable ignores with a --ignore option #36 : files to ignore in a scan
• Include license policy using a configuration file #214 : license policy choices
• Proposal: Scan deduction and summarization #377 : deduction and summarization preferences and options
• Filter out/ignore copyrights by a specific author/holder #106 : define copyright holders to be skipped
• License data should have pointers to next and previous versions for versioned licenses #125 and License data for exceptions should have pointers the main license the exception relates to #126 : set a preferred license choice when there is a choice or an "or later version" license

This is also related to these tickets:

• Simplify the scancode command line options #209 : simplify CLI options
• Provide option to reference a specific DejaCode dataspace in scan results #280 dejacode options
• Implement an option for shallow extraction #262 ways and options to collect license texts
• Limit number of urls and emails reported in a single file  #384 Limit number of urls and emails reported in a scan for a single file
• Scan output doesn't came up with a root directory #447 stripping root directory or not
• Option to only report first occurrence of a license of a kind #464 report only the first occurrence of a license