Speedup startup on Cygwin by avoiding costly system() call.

By chance I discovered during profiling the Vim startup that open-browser was in the top 3 of Vim startup time. Analysis showed that this was caused by an unnecessary system('uname') call in the Mac detection; forking a new process is especially slow on Cygwin, due to the architecture mismatch with the underlying Windows subsystem.
Merge pull request #13 from inkarkat/cygwin-speedup

Speedup startup on Cygwin by avoiding costly system() call.
fix #14: On mswin, URL encode may result in unexpected expansion by environment variable
re-fix #14: change default value of g:openbrowser_open_rules (thanks @Milly)
refactoring: add a function to detect a query is uri,filepath,etc.
chmod -x {plugin,autoload}/openbrowser.vim
cosmetic fix
reduce startuptime to avoid calling system().
remove duplicated URI-escape

openbrowser#open() does URI-escape.
So openbrowser#search() does not need to do URI-escape.
use extend() and get() for global variable initialization
:OpenBrowser should support expand() argument
use vimproc#system() for speed optimization if it exists
Revert ":OpenBrowser should support expand() argument"

This reverts commit f2f506fc4919e39d60d58f680a62dcab943d3862.

This commit may result in unexpected expansion in URI.