Add hash rocket '=>' token

[aycabta]

You can check this feature as sample file below:

# sample code:
#
#     { :a => :b }
#
def meth
end

The original RDoc generates:

<pre class="ruby">
{ <span class="ruby-value">:a</span> =<span class="ruby-operator">&gt;</span> <span class="ruby-value">:b</span> }
</pre>

This separated between = and > as different token types.

RDoc with this Pull Request generates:

<pre class="ruby">
{ <span class="ruby-value">:a</span> <span class="ruby-operator">=&gt;</span> <span class="ruby-value">:b</span> }
</pre>

This provides => as one token.

[hsbt]

It seems reasonable.