Add CDATA escaping to exml

[NelsonVides]

Expose a way to select cdata to be printed as an escaped string or as a CDATA[] section. The latter can make the string considerably smaller when there is a lot of information that needs to be escaped, for example when printing URLs.

So that this package:

<name>
  <![CDATA[{"one":1,"two":"bis","three":["raz","dwa","trzy"]}]]>
</name>

Doesn’t become this one when printing (which is not only unreadable but also bigger):

<name>
  {&quot;one&quot;:1,&quot;three&quot;:[&quot;raz&quot;,&quot;dwa&quot;,&quot;trzy&quot;],&quot
;two&quot;:&quot;bis&quot;}
</name>

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.

Project coverage is 73.94%. Comparing base (79e0f21) to head (70328df).

Files with missing lines	Patch %	Lines
c_src/rapidxml_print.hpp	40.00%	6 Missing ⚠️
c_src/exml.cpp	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   72.20%   73.94%   +1.73%     
==========================================
  Files           7        7              
  Lines        1173     1182       +9     
  Branches      167      169       +2     
==========================================
+ Hits          847      874      +27     
+ Misses        326      308      -18     

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[JanuszJakubiec]

Looks good 👍🏻