Expand functionality of <CR> mapping

[PhilRunninger]

Description of Changes

Closes #551
Closes #631

This change creates a customizable key mapping that can open files or directories in a way that is different from the normal commands. By default, it is set to <CR>, the Enter key, and its action is to do the same thing as the o key. But the key assigned to this action, and the options given to the internal NERDTree open functions, can be changed. The documentation describes the values that can be set in these options.

This pull request addresses two outstanding issues.

#551 - Use Enter to open a file in a new tab. To do this, add the following to your .vimrc.

let NERDTreeCustomOpenArgs = {'file':{'where':'t'}}

#631 - Use Enter to open a file in the current tab, even if open in another one. This requires the reuse flag to be overridden, and is done with this code in your .vimrc.

let NERDTreeCustomOpenArgs = {'file': {'reuse':'currenttab', 'where':'p', 'keepopen':1, 'stay':1}}

Many other such customizations are possible, and are documented in the NERDTree help file.

---

New Version Info

• Derive a new version number. Increment the:
  • MAJOR version when you make incompatible API changes,
  • MINOR version when you add functionality in a backwards-compatible manner, and
  • PATCH version when you make backwards-compatible bug fixes.
• Update version number in autoload/nerdtree.vim
• Update the ChangeLog, following this format/example:

  MAJOR.MINOR...
  - **.PATCH**: PR Title (Author) #PR Number
  
  ### 5.1...
  - **.1**: Update Changelog and create PR Template (PhilRunninger) #1007
  - **.0**: Too many changes for one patch...
  

[andykog]

@PhilRunninger, started getting this error on attempt to open my bookmarks:

E121: Undefined variable: node
Error detected while processing function nerdtree#ui_glue#invokeKeyMap[1]..82[43]..81[3]..<SNR>73_customOpenBookmark:

Vim info:

VIM - Vi IMproved 8.1 (2018 May 18, compiled Feb  5 2019 07:31:26)
macOS version
Included patches: 1-873
Compiled by Homebrew
Huge version with MacVim GUI.  Features included (+) or not (-):
+acl               +cmdline_hist      +ex_extra          +jumplist          +mouse_dec         +path_extra        +startuptime       +title             -X11
+arabic            +cmdline_info      +extra_search      +keymap            -mouse_gpm         +perl              +statusline        +toolbar           -xfontset
+autocmd           +comments          +farsi             +lambda            -mouse_jsbterm     +persistent_undo   -sun_workshop      +transparency      +xim
+autochdir         +conceal           +file_in_path      +langmap           +mouse_netterm     +postscript        +syntax            +user_commands     -xpm
-autoservername    +cryptv            +find_in_path      +libcall           +mouse_sgr         +printer           +tag_binary        +vartabs           -xsmp
+balloon_eval      +cscope            +float             +linebreak         -mouse_sysmouse    +profile           +tag_old_static    +vertsplit         -xterm_clipboard
+balloon_eval_term +cursorbind        +folding           +lispindent        +mouse_urxvt       -python            -tag_any_white     +virtualedit       -xterm_save
+browse            +cursorshape       -footer            +listcmds          +mouse_xterm       +python3           +tcl               +visual
++builtin_terms    +dialog_con_gui    +fork()            +localmap          +multi_byte        +quickfix          +termguicolors     +visualextra
+byte_offset       +diff              +fullscreen        +lua               +multi_lang        +reltime           +terminal          +viminfo
+channel           +digraphs          -gettext           +menu              -mzscheme          +rightleft         +terminfo          +vreplace
+cindent           +dnd               -hangul_input      +mksession         +netbeans_intg     +ruby              +termresponse      +wildignore
+clientserver      -ebcdic            +iconv             +modify_fname      +num64             +scrollbind        +textobjects       +wildmenu
+clipboard         +emacs_tags        +insert_expand     +mouse             +odbeditor         +signs             +textprop          +windows
+cmdline_compl     +eval              +job               +mouseshape        +packages          +smartindent       +timers            +writebackup

[PhilRunninger]

@andykog , This was discovered about 4 hours ago, and PR #1014 was submitted. I'll get the fix out soon.

[andykog]

Cool, thank you very much!