Run content [#19]

- [x] Add bidirectional override (a)
- [x] Add bidirectional embedding level (b)
- [x] Add content part (c)
- [x] Add control (d)
- [x] Add carriage return (e)
- [x] Add long day (f)
- [x] Add short day (g)
- [x] Add deleted text (h)
- [x] Add invalidated field cache (i)
- [x] Add phonetic guide text size (k)
- [x] Add phonetic guide base text size (l)
- [x] Add phonetic guide text size rasie (m)
- [x] Add position of last rendered page break (n)
- [x] Add language id for phonetic guide (o)
- [x] Add long month (p)
- [x] Add short month (q)
- [x] Add non-breaking hyphen (s)
- [x] Add page number block (v)
- [x] Add absolute position tab character (w)
- [x] Add phonetic guide text (x)
- [x] Add phonetic guide (y)
- [x] Add phonetic guide text alignment (z)
- [x] Add phonetic guide text base (aa)
- [x] Add phonetic guide properties (ab)
- [x] Add optional hyphen (ac)
- [x] Add tab (ad)
- [x] Add long year (ae)
- [x] Add short year (af)
You're right -- it says
```
     Failure/Error: expect do
       expected no Exception, got #<TypeError: nil is not a symbol> with backtrace:
         # ./lib/rocx/attribute_builder.rb:212:in `block in attribute'
         # ./spec/support/element_test_macros.rb:88:in `block (2 levels) in it_should_assign_successfully'
         # ./spec/support/element_test_macros.rb:86:in `block in it_should_assign_successfully'
     # ./spec/support/element_test_macros.rb:86:in `block in it_should_assign_successfully'
```
which is a bit confusing unless you're used to it.

I'm not sure how I want to proceed with that -- I could either define a default like `expects ||= :true_or_false` or raise an Exception `raise ArgumentError if expects.nil?`

I guess alternatively I could bump the minimum version of Ruby to 2.1, which has a concept of _required_ named arguments.

What do you think?