Allow non-alphanum characters in tag content

[xueweiz]

The tag content should allow non-alphanum characters. See the spec for section tag content [1]:

These tags' content MUST be a non-whitespace character sequence NOT containing the current closing delimiter; ...

For the 3 added test cases, #1 and #3 will fail today, and #2 will trigger a panic. The code change fixes them.

The expected behavior can also be verified on http://mustache.github.io/#demo, with Mustache:

1: {{#key*}}{{.}}{{/key*}}
2: {{#key}}{{*}}{{/key}}
2: {{#key}}{{*}*}}{{/key}}

and JSON:

{
  "key*": "value*",
  "key": "value",
  "*": "star",
  "*}*": "fish"
}

We can get output as:

1: value*
2: star
2: fish

[1] https://github.com/mustache/spec/blob/b1329a25e6d265ff360267d23f7c6327bbf59f52/specs/sections.yml#L5

[xueweiz]

@alexkappa I understand that you are very busy, I'm not trying to push you :) Just want to let you know that the change is ready for review. Happy to wait until your convenience. Thanks!