Rename x_orig to xOrig

As suggested by golint. Underscores aren't very go-ish.
Replace zero-length slice with nil

This avoids an unnecessary call to make. As suggested by the new golint.
Add TerminalSupported

This function returns true if line editing is available, and false if
the fallback (no line editing) will be used.
Add function TerminalMode and interface ModeApplier

These functions are provided as a convenience for the management of the
terminal input mode state. Most users of liner will never need to call
them directly.
Fix TERM=dumb

Only check ANSI/ECMA-48 capabilities if the terminal is not blacklisted.
Support alternate Home/End encoding

This encoding is used by rxvt-unicode
Support alternate encoding of Ctrl-Left and -Right

This encoding is used by rxvt-unicode
Don't try to detect CHA support with DSR/CPR

Not all terminals support ANSI DSR/CPR (for example, the telnet.exe that
comes with Windows, despite reporting itself as TERM=ansi).
Merge branch 'master' of https://github.com/peterh/liner

Conflicts:
	input.go
Merge branch 'master' of https://github.com/peterh/liner