find_alias_in_file: Return a vector of matches

Instead of returning a boolean value, get all lines that include the
alias being searched for. This will give us a list of values like:

    Ok([
        "alias farnsworth-hubert Hubert Farnsworth <professor@planetexpress.com>",
        "alias farnsworth-hubert-2 Hubert Farnsworth <other@planetexpress.com>",
        "alias farnsworth-hubert-3 Hubert Farnsworth <other2@planetexpress.com>"
    ])

Our list will contain all Mutt alias calls that start with the given
alias string.

Use my new learnings from the Error Handling section of the Rust book
(http://doc.rust-lang.org/book/error-handling.html). We return a Result
type so that we can communicate both File IO errors and our own custom
matching errors. We define a new error type that allows us to provide
information about the reason for our own errors.

Note that I haven't yet implemented the required `fmt::Display` and
`error::Error` methods on my custom error type yet as those didn't
appear to be crucial to just getting this working.

Change the `line.unwrap()` line to `try!(line)` so that we don't panic
in the event of an error, but instead bubble the error upstream.

If we come across a Mutt alias that has the same email as the inputted
search alias, we want to abort because we shouldn't add a new Mutt alias
line for an existing email address that's already been aliased.

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