allow to pass a custom divider that is used to join grouped lines

Currently the grouped lines of tags are joined by an space character. In my implementation for "Deutsche Bank" I would need to join them without any divider or with an empty string.

This pull request adds a new parameter `join_lines_by` to `FileContent.new` that allows to specify a custom divider, e.g.

```ruby
@bank_statements = MT940Structured::Parser.parse_mt940(@file_name, "")
```

or

```ruby
@bank_statements = MT940Structured::Parser.parse_mt940(@file_name, "\n")
```