Turn a series of strings into options exposed as an iterator.

When parsed, input strings are determined to be one of two types: a name or a value. From the sequence of names and values, the type of option is determined. It is one of:

• Flag is a name without a value
• Ordinal is a value without a name
• Named is a value with a name
• Raw is simply a raw input string

Names come in two different types: short (1 char) and long (2+ chars). Long names start with -- and short names with -. Short name Flags can be stacked, for example -abc represents the flags a, b, and c.

- alone is treated as a value. -- signifies the end of options and is not emitted. All strings after -- are passed along as a Raw option.

opter consumes iterators over strings, or Iterator<Item = String>. To use opter, pass a struct that has an implementation for IntoIterator to opter::parse. Parsing command-line options is a common use case that is met by opter::parse_env.