Improve parse item with multi links

[RoCry]

Some rss like https://www.coindesk.com/arc/outboundfeeds/rss/?outputType=xml will have multi links in single item.

So we could improve parse it by just keep the first valid link.

Let me know if you guys have better way to handle this.

[codecov[bot]]

Codecov Report

Merging #139 (dd27c2b) into master (878bb2d) will increase coverage by 40.68%. The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           master     #139       +/-   ##
===========================================
+ Coverage   45.14%   85.82%   +40.68%     
===========================================
  Files          26       26               
  Lines        4357     2145     -2212     
===========================================
- Hits         1967     1841      -126     
+ Misses       2390      304     -2086     

Impacted Files	Coverage Δ
src/item.rs	89.94% <60.00%> (-0.97%)	:arrow_down:
tests/read.rs	100.00% <100.00%> (ø)
src/error.rs	0.00% <0.00%> (ø)
src/toxml.rs	100.00% <0.00%> (+27.77%)	:arrow_up:
src/extension/mod.rs	68.00% <0.00%> (+28.46%)	:arrow_up:
src/category.rs	93.33% <0.00%> (+35.00%)	:arrow_up:
src/lib.rs	66.66% <0.00%> (+59.78%)	:arrow_up:
src/util.rs	80.00% <0.00%> (+68.88%)	:arrow_up:
src/source.rs	96.55% <0.00%> (+75.49%)	:arrow_up:

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