[0.8] Advanced TS types

[jaredpalmer]

Adds more expressive type signatures to FormikError and FormikTouched. TypeScript will now get angry if you try to set a field on errors or touched that does not exist on values.

Old

interface FormikValues {	
 [field: string]: any;		
}

interface FormikErrors {	
 [field: string]: string;		
}

interface FormikTouched {	
 [field: string]: boolean;		
}

New

export interface FormikValues {
  [field: string]: any;
}

export type FormikErrors<Values extends FormikValues> = {
  [Key in keyof Values]?: string
};

export type FormikTouched<Values extends FormikValues> = {
  [Key in keyof Values]?: boolean
};