Adds support for tag synonyms

[ryanoasis]

• currently for: @returns, @param, @function, @class
• originally discussed on PR #23 (... plural form of '@return')
• makes '@returns' the default per JSDoc

Not sure if/how you want to version bump, also this would change the default from 'return' to 'returns' so I am not sure if you want to change the default back (I went with what looked to be the default on JSDoc website).

I tested a few method types with and without some vimrc settings:

" test synonyms
let g:jsdoc_tags = {}
let g:jsdoc_tags['param'] = 'arg'
let g:jsdoc_tags['returns'] = 'return'

[heavenshell]

@ryanoasis Thank you so much!! I'll look your PR this week end.

[ryanoasis]

No problem and no rush. Thanks :smile:

[heavenshell]

@ryanoasis Very nice!! Thank you so much!