Expand functionality of <CR> mapping (#1011)

* Define default values for new variables governing new "Default Open".

"Default Open" means to open a file with the Enter key (which of course
can be changed.) The key can be changed in the vimrc, and there is a
variable for specifying the Opener parameters for opening the node. This
lets the user decide whether Enter (or another key) will open a file in
the current tab or a new one, and whether or not to open the file again
in the tab or jump to a window where the file is already open.

* Remove the old mapping for <CR>, a duplicate of 'o'.

<CR> will be defined like all the other keys, in plugin/NERD_tree.vim

* Assign functions to the new <CR> key mapping.

Three separate functions handle directories, files, and bookmarks.

* Rename variables: NERDTreeCustomOpen and NERDTreeCustomOpenArgs

* Add documentation for NERDTreeCustomOpenArgs and NERDTree-<CR>.

* Make key mapping variables be indexed in the Vim help

* Remove angle brackets from <CR> to see if help navigation improves.

* Rename functions from defaultOpen... to customOpen...

* Use separate options for file and directory nodes.

* Update documentation for separate file/directory options.

* Update version number and change log.

* Change CR to <CR> in help tags.

* Fix missing backtick in patch number.

* Update the quickhelp text.

* Update Pull Request template.

* Update change log with simpler formatting of patch number.

* Get NERDTree version number directly from CHANGELOG.md

* Reformat the lists of contributors in the Change Log.

* Initialize the version text, just in case the while loop finds nothing.

.github/ISSUE_TEMPLATE/bug.md

@@ -23,7 +23,7 @@ After reading, and before submitting your issue, please remove this introductory
 #### Environment (for bug reports)
 - [ ] Operating System: 
 - [ ] Vim/Neovim version `:version`: 
-- [ ] NERDTree version `git rev-parse --short HEAD`: 
+- [ ] NERDTree version `:echo nerdtree#version(0)` or `git rev-parse --short HEAD`: 
 - [ ] A link to my [vimrc](), or
 - [ ] vimrc settings
     - [ ] NERDTree variables