(New validator) Validate non-existing but valid file/directory paths

[johnnybubonic]

NOTE: Because these are added as new validators, file and dir validator behavior remains the same/backwards compatibility is maintained.

Fixes Or Enhances

Fixes #808

• Adds filepath and dirpath validators (for testing syntax validity of a path but not the required existence of it)
• Slight modifications to file and dir validator documentation to disambiguate from the above.
• Minor gofmt-ing (thanks, GoLand.)

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

Coverage: 74.273% (+0.01%) from 74.262% when pulling 55f7244 on johnnybubonic:master into f560fd4 on go-playground:master.