Add auto-configuration for Cygwin

It is automatically enabled when the OS name is reported as
CYGWIN_NT-6.1 and will detect and inform the user whether the ncurses
and iconv packages need to be installed.

Tested on Windows 7 with the currently available 64-bit Cygwin version.
Load HEAD ID and symbolic name as part of the repo info

This ensures that opt_remote is populated before the git config is
loaded so that tracking information for the current branch can be used
when highlighting remote references.
Optionally load HEAD when loading references

This avoids one fork+exec and improves the start-up time on Cygwin.

References #92
Disabled colors last when passing user supplied arguments to Git

This fixes situations where colors are enabled by for example
TIG_DIFF_OPTS:

	TIG_DIFF_OPTS="--word-diff=color" tig

Fixes #191
Add readline support (WIP)
readline: add variable completion