added function getAvailablePropertyNames (fixes iss #44) and rewrote function getNRandomCountries (fixes iss #70) and wrote function getCountryDetailsByPhoneCode (fixes iss #71)

[AliLordLoss]

General

Reference: <#44>
Description:

added a new function to index.js file

Type of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have updated the documentation accordingly
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed

Additional notes