Pass v:lnum as an argument to GetJsonIndent

[Ajhad1]

Updated to allow for better debugging. The indent script should work as it did before, but you'll be able to call the function manually with a line number.

Based on similar commit for vim-ruby https://github.com/vim-ruby/vim-ruby/commit/bcdd3f93e7c4c1aec02131ea8bf9c5afe07bf767