Moved the code to find word under cursor to parent.

The get_current_word function calculates the word under cursor.

* If params are present they are used
* If selection is present, uses get_selected_text by @xolox
* Else <cword> is used.
Adds GotoDefinitionCommand and tests.

* :Wdef keyword - will jump to the keyword under cursor, selection, etc.
* The <c-]> and <c-w>] mappings now use :Wdef instead
* This implementation filters out stub tags by using :{count}tag jumps.
  The actual order of tags is still the same.
* Some of this refactoring eases the wpseek API implementation.
Merges branch 'feature/add-goto-definition-command' into develop
Adds ChoicePrompter.

API is `prompter.prompt(choices)`

Returns,
-1 : quit
1-N : index of choice made
Adds is_false and is_true helpers.
Fixed return from main api function.
Adds g:wordpress_vim_path to main plugin.

This path is used to resolve the path to the wpseek python api.
Tests for is_true & is_false helpers
Adds wpseek pattern.
Adds WpSeek python api.
Adds WpSeekApi & helpers.
Adds SimilarFunctionsCommand.

The excommand `:Wsimilar` will now lookup similar functions for word
under cursor, selection etc. Uses the WpSeek.com API.
Adds SimilarTopicsCommand.

The ex-command `:Wtopics` shows topic similar to word under cursor,
selection, etc. Use the WpSeek.com API.
Adds WpSeek commands to controller.
Improves Jump to Core documentation.

* Adds docs for g:wordpress_vim_jump_to_core_mappings
Adds test for disabled jump to core mappings.

Improves documentation for Jump to Core.
Adds documentation for WpSeek integration.
Marks WpSeek Todo as DONE.
Merges branch 'feature/add-Wsimilar-command' into develop
Bumps version to 0.1.5