Added missing AskOpts

[AlecAivazis]

This PR continues the work in #201 and adds the remaining AskOpts:

• survey.WithHelpInput to change the rune that prompts look for to provide help.
• survey.WithIcons to change the icons
• survey.WithFilter to change the default filter applied to Select/MultiSelect

This has to go in after #214 since it relies on the PromptConfig which was introduced it that PR.

[AlecAivazis]

@coryb there is now parity across the methods in the Prompt interface. Not sure why I wanted to avoid another file in the root of the project when there are already so many...

[AlecAivazis]

I thought a bit more about this API and i think one of two things needs to happen. Either I should remove WithHelpInput or HelpInput should be removed from the IconSet struct. It's not clear from the API what would happen if you passed both WithHelpInput and a WithIconSet that modified the icon since they modify the same field in the icon set.

Any preference @coryb? Keeping it in the IconSet has a smaller API surface since its kind of tucked away. But it's also not really an "icon" like the others since it's input and would never include any kind of color configuration regardless of how #141 shakes out.

EDIT: I decided to go with removing HelpInput from the IconSet. I think this is good to go now.

[coryb]

looks good, I agree with the HelpInput change and moving the Icons to the Config struct.