Added support for --variable=V option.

This option, present in pkg-config will print to stdout defined variable
inside .pc file. This variable can be anything, from user specified to
standard ones, like 'cflags' or 'libs'.
Behave just like pkg-config.

When variable wasn't found, pkg-config will output newline without
warnings. The same is applied here.
Added options --libs-only-L and --libs-only-l. Documenting --variable.
Assure Getopt::Long does not ignore case; this affects --libs-only-l and
--libs-only-L.
Watch how --libs-only-L and --libs-only-l behaves in combination.
Removed unused import.
Allow -R to be recognized for --libs-only-L
remove tabs
Merge pull request #5 from plicease/variable_option2

Support for --variable (no tabs)