Added option to prevent automatic C-l mapping (default remains same)

[faceleg]

No description provided.

[heavenshell]

Thanks for pull request.

I've got an error.

Error detected while processing /tmp/vim-jsdoc/ftplugin/javascript/jsdoc.vim:
line   15:
E121: Undefined variable: g:jsdoc_default_mapping
E15: Invalid expression: !hasmapto('<Plug>(jsdoc)') && g:jsdoc_default_mapping

I did not set g:jsdoc_default_mapping in my .vimrc.
IMO initializing g:jsdoc_default_mapping should be do in ftplugin/javascript/jsdoc.vim.
What do you think?

If error solved, it's ok to merge.

[faceleg]

My fault, you're right - I've fixed it.

[heavenshell]

@faceleg Merged your PR.
Thank you for making jsdoc.vim better.