Adds support for tag synonyms

[ryanoasis]

• currently for: @returns, @param, @function, @Class
• originally discussed on PR Enable user to use the plural form of @return #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 😄

[heavenshell]

@ryanoasis
Very nice!!
Thank you so much!