Add hreflang meta generation

[batopa]

This PR add the ability to generate hreflang meta tag for i18n pages.

To use it in view:

echo $this->I18n->metaHreflang();

For example the generated HTML for an URL as https://example.com/en/help will be

<link href="http://localhost/en/help" rel="alternate" hreflang="en"/>
<link href="http://localhost/it/help" rel="alternate" hreflang="it"/>

for I18n.languages configuration

Configure::write('I18n', [
    // other I18n stuff
    // ....
    'languages' => [
        'en' => 'English',
        'it' => 'Italiano',
    ],
    // other I18n stuff
    // ....
]);

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 6.12%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #18      +/-   ##
============================================
+ Coverage     39.11%   45.23%   +6.12%     
- Complexity      101      106       +5     
============================================
  Files             5        5              
  Lines           317      336      +19     
============================================
+ Hits            124      152      +28     
+ Misses          193      184       -9

Impacted Files	Coverage Δ	Complexity Δ
src/View/Helper/I18nHelper.php	97.33% <100%> (+16.97%)	33 <6> (+5)	⬆️

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