Add toggle for multiple select and removable tags

[michaeltheshah]

This pull request addresses issue #100. It adds a boolean toggle named allowMultipleSelection that allows a user to select multiple tags at once.

let cities = ["London", "New York", "Chicago"]
let tagField = WSTagsField()
tagField.allowMultipleSelection = true

Also, a boolean toggle named removable is added that doesn't allow a tag to be removed. This is inside WSTagView and set with a setter named setRemovable(tags: [String], removable: Bool = false). This method will throw an exception if called before any tags are added.

let cities = ["London", "New York", "Chicago"]
let tagField = WSTagsField()
tagField.addTags(cities)
tagField.setRemovable(cities) //removable set to false by default

Finally, a helper function is created named getSelectedTagStrings() that gets all tags as a list of strings.

let cities = ["London", "New York", "Chicago"]
let tagField = WSTagsField()
tagField.getSelectedTagStrings()

[ricardopereira]

@mochat97 Nice one, thank you. I'll test this and review it shortly. Exciting!

[ricardopereira]

Hmm, I think we should do another approach. This could not always solve the problem.

[ricardopereira]

Please try to find another way avoiding the use of asserts. This is quite unsafe in runtime.

[ricardopereira]

This should not be open, we don't want to expose this to lib users.

[ricardopereira]

Please provide a functional code in the Example app. Thank you for your contribution.