Allows specs to be an array of (optionally colon-delimited) values

Allow argument to extract_marc (actually, parse_string_spec) to take an array or strings. Also relaxes the colon-delimited requirement to allow spaces around the colons.


This will allow stuff like the following:

```ruby
to_field 'foo', extract_marc %w(
        245abcde
        810
        700|*4|bcd
)

to_field('bar'), extract_marc %w( 
       245abcde
       100:110:111
       810
       700|*4|bcd
)

```