Close #1018

Run the command as is in cmd.exe with no parsing and escaping.
Explicity set cmd.SysProcAttr so execCommand does not escape the command.
Technically, the command should be escaped with ^ for special characters,
including ". This allows cmd.exe commands to be chained together.

See https://github.com/neovim/neovim/pull/7343#issuecomment-333350201

This commit also updates quoteEntry to use strings.Replace instead of
strconv.Quote which escapes more than \ and ".

- Use bash for `set -o pipefail`
- Fall back to simpler find command when the original command failed

Related: #1061

- Update install script to download Windows binary if $TERM == cygwin
- Unset TERM if $TERM == cygwin (#933)
- Always use cmd.exe instead of $SHELL when running commands

Due to go std lib uses poller for os.File introducing in this commit:
https://github.com/golang/go/commit/c05b06a12d005f50e4776095a60d6bd9c2c91fac


There are two changes to watch out:
1. os.File.Fd will always return a blocking fd except on bsd.
2. os.File.Read won't return EAGAIN error for nonblocking fd.

So
For 1, we just get tty's fd in advance and then set its block mode.
For 2, we use read syscall directly to get what we wanted error(EAGAIN).

Fix issue #910.

Signed-off-by: Tw <tw19881113@gmail.com>

- Fix display of CJK wide characters
- Fix horizontal offset of header lines
- Add support for keys with ALT modifier, shift-tab, page-up and down
- Fix util.ExecCommand to properly parse command-line arguments
- Fix redraw on resize
- Implement Pause/Resume for execute action
- Remove runtime check of GOOS
- Change exit status to 2 when tcell failed to start
- TBD: Travis CI build for tcell renderer
    - Pending. tcell cannot reliably ingest keys from tmux send-keys

`TAGS=termbox make` (or `go build -tags termbox`)