Support for gabrielelana/vim-markdown

Added support for gabrielelana/vim-markdown, a third alternative for using markdown in Vim.

Further adjustments by comparing directly with iA Writer.
Support for gabrielelana/vim-markdown (2)
Support very subtle background for code blocks and inline

Inspired by how code appears in the markdown at github.
Support Conceal feature w/tpope's markdown
sync guy and term attributes; add italic support
Improve docs for Conceal support
Reduce contrast of '~' at end of file

The NonText highlight controls the display of the tildes at the end of the file.

Like other indicators, it can be distracting when editing, so likewise reducing its contrast.
Ensure escaped chars match normal text

To match iAWriter behavior
More details on using Vim's conceal feature
Linked to @gummesson's terminal support for pencil
Support for mattly/vim-markdown-enhancements
Removed code background option

Thinking about how best to support feature variants of this color scheme.
Refactoring to links
Temporarily back out of refactoring changes to fix #11
Disable terminal italics by default, opt-in w/ var

Yes, terminal emulation is a ghetto.  Why should that surprise you?
Merge remote-tracking branch 'temp/master'