Add an option for an activation char for special attributes.

[NEricN]

Sometimes, the current special attribute model can lead to accidental attributes made with brackets. The behavior is currently not configurable. This change allows the special attribute to require a specified char in front of special attributes, to make it harder to make it accidental.

For example, if the char was set to :, then # h {:.class} would turn into <h1 class="class">h</h1> but # h {.class} would not.

[codecov-io]

Current coverage is 62.46% (diff: 71.42%)

Merging #36 into master will increase coverage by 0.24%

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  Files            14         14          
  Lines          3880       3879     -1   
  Methods         255        256     +1   
  Messages          0          0          
  Branches       1044       1042     -2   
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+ Hits           2414       2423     +9   
+ Misses         1028       1024     -4   
+ Partials        438        432     -6   

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[kjdev]

Thanks.