Support for multiple `@` characters

X-Reference from [FTPClient.jl](https://github.com/invenia/FTPClient.jl/issues/88)

When using multiple `@` characters URIParser fails with various errors depending on the amount of `@` characters passed in.

Examples:

```julia
using URIParser

URI("ftp://user@example.com:password@example.com")

> ERROR: Port must be numeric (decimal)
```

```julia
using URIParser

URI("ftp://user@example.com:p@ssword@example.com")

> ERROR: Port must be numeric (decimal)
```

```julia
using URIParser

URI("ftp://user:p@ssword@example.com")

> ERROR: Unexpected character '@' in host
```