Add helper function to transform string states into int states

[martialblog]

This PR adds a helper function to transform string states into int states.
It's the invert of the existing StatusText.

Unsure if this should should return an error if "invalid input" is given, the StatusInt doesn't so I decided to go for consistency.

While writing this, I thought: is there a reason we don't have a "Status" type with these methods attached? 🤔

See #125

[martialblog]

Do we gain anything from a custom Int type?
Implement interfaces like Stringer yes... but otherwise?

type Status int

// String returns the string corresponding to a state
func (s Status) String() string {
	switch s {
	case OK:
		return OKString
	case Warning:
		return WarningString
	case Critical:
		return CriticalString
	case Unknown:
	}

	return UnknownString
}

// NewStatusFromString returns a state corresponding to its
// common string representation
func NewStatusFromString(status string) Status {
...

[RincewindsHat]

The thought of a Status type has occured to me and I do tend to like it, since the status is semantically not a number and it would improve the clarity of the code in here to handle status differently from numbers.

Therefore I am for introducing a specific status type

[RincewindsHat]

I do understand the wish for consistency, but IMHO there is a clear error case here and I feel uncomfortable with the function handling this the way it is now.

I think I would be rather (unpleasantly) surprised to find out, that this function transforms "Critiacl" (a typo of "Critical") to Unknown without giving me a way to verify it.

And yes, the function is rather trivial and I might be heavily prone to overengineering.