Add a command to add a greeting text to the Facebook messenger app

[christophrumpel]

This PR adds an Artisan command to set the messengers app greeting text.

[codecov]

Codecov Report

Merging #11 into master will decrease coverage by 2.33%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master      #11      +/-   ##
============================================
- Coverage     75.21%   72.88%   -2.34%     
- Complexity      267      271       +4     
============================================
  Files            27       28       +1     
  Lines           686      708      +22     
============================================
  Hits            516      516              
- Misses          170      192      +22

Impacted Files	Coverage Δ	Complexity Δ
src/Commands/AddGreetingText.php	0% <0%> (ø)	4 <4> (?)

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