Use lines numbers to jump to the exact match line

Fixes #2. Also general unite options style changes

1) Changed g:unite_codesearch_command to
   g:unite_source_codesearch_command

   This should only contain the binary name without any flag as we
   need to control the -i -n -m "%s" flags.

2) Introduces max_candidates

   Use via call unite#custom#source('codesearch', 'max_candidates', 20)

3) Introduces ignore_case

   Use

   let g:unite_source_codesearch_ignore_case = 1