add helper mappings for a number of features

Includes some helper mappings I find useful.  Feel free to reject this PR but as long as I was adding them to my fork, I thought I'd also offer them upstream.

Shortcuts
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### ESC
`jk` is mapped to ESC in visual mode

### Tab Settings
set number of characters and treatment (spaces or \t)
`<leader>ts4` " i.e. (t)ab as (s)paces, 4 spaces
`<leader>ts2` " i.e. (t)ab as (s)paces, 2 space
`<leader>tt4` " i.e. (t)ab as (t)ab character, display size 4
`<leader>tt2` " i.e. (t)ab as (t)ab character, display size 4

### Modify .vimrc and .gvimrc on the fly
`<leader>ev` edit .vimrc file
`<leader>sv` source .vimrc file (i.e. apply settings in that file)

`<leader>eg` edit .gvimrc file
`<leader>sg` source .gvimrc file (i.e. apply settings in that file)

### PHP error logging
`<leader>el` the current expression on a line and error log it with comment
**$str** becomes 
**error_log( '$str = ' . print_r( $str, true ) );**

### Remove Whitespace
`<leader>w` clean all trailing whitespace