Add customization group for github-browse-file
Extract helper for calculating lines for anchor

Will allow adding an option for showing line at current point by
default.
Add customization to show line at point

If customization is enabled will add an anchor linking to current line
by default.

Relates to ticket #2.
Customization to only add url to kill ring

This allows the user to change the default to only add the url to the
kill ring, and display the URL instead of calling browse-url.

Potential solution to ticket #3
Merge branch 'customize-showing-line-at-point'

Conflicts:
	github-browse-file.el