Add ability to pass a transformation block to element_counts

[Fryguy]

@bdunne Please review. I found I needed this for the memory_analyzer. Makes counting objects by arbitrary properties a lot easier.

[bdunne]

👍 LGTM, merge when green

[miq-bot]

Checked commit Fryguy@2f3667d with ruby 1.9.3, rubocop 0.34.2, and haml-lint 0.13.0
2 files checked, 5 offenses detected

spec/core_ext/array/element_counts_spec.rb

• 🔶 - Line 5, Col 40 - Style/BracesAroundHashParameters - Redundant curly braces around a hash parameter.
• 🔶 - Line 6, Col 42 - Style/BracesAroundHashParameters - Redundant curly braces around a hash parameter.
• 🔶 - Line 7, Col 55 - Style/BracesAroundHashParameters - Redundant curly braces around a hash parameter.
• 🔶 - Line 12, Col 50 - Style/BracesAroundHashParameters - Redundant curly braces around a hash parameter.
• 🔶 - Line 13, Col 65 - Style/BracesAroundHashParameters - Redundant curly braces around a hash parameter.