[Yaml-core] excluding '<' in ns-first-plain-char (for XML auto-detection)

[Clark C. E. <cc...@cl...>]

I was just reading the parrot list, and noticed that they were 
including YAML in their serialization plans -- which is great.  
However, they were also trying to mix it with XML (I'm going to 
post a follow up to that list pointing them to http://yaml.org/xml/).  

Anyway, this reminded me that we had restriction early on that 
allowed us to 'auto-detect' XML streams so that some tools could
'automap' them onto YAML.  To enable this feature, we had only to
restrict starting mapping keys with the '<' character.   Anyway,
I think (perhaps I'm wrong here) that the current specification
dropped this restriction recently... and I'm suggesting that we
add it back:

   ie, alter ns-first-plain-char to exclude '<'

Best,

Clark