add .gitignore

specifically, this is intended to avoid the repo becoming dirty
when the help file doc/tags is autogenerated. I'm using
Pathogen and using open-browser as a git submodule,
when I run `:Helptags` it generates doc/tags. As a note, I've noticed some
of the other vim plugin repos I have as submodules are using a .gitignore
file in this way.
Merge pull request #46 from salcode/gitignore

add .gitignore
new variable: g:openbrowser_message
replace g:openbrowser_message with g:openbrowser_show_message
Merge pull request #47 from itchyny/master

new variable: g:openbrowser_message
strip new line characters (fix #48)
check opening text is exactly under cursor (fix #49)
Merge branch 'check-under-cursor'
Merge pull request #50 from itchyny/master

strip new line characters (fix #48)
continuous newlines -> single whitespace (#50)
merge two variables to g:openbrowser_format_message (#47)

Now the following variables are merged to g:openbrowser_format_message.
* g:openbrowser_show_message
* g:openbrowser_short_message

This commit also improves s:expand_keywords() function.
Now the function supports nested expression like
g:openbrowser_format_message default value:
    "opening '{uri}' ... {done ? 'done! ({command})' : ''}"
Merge branch 'format-message-variable'
negate col in openbrowser#get_url_on_cursor() (fix #49)
calculate right col (fix #49)
Merge branch 'itchyny-49-fix' (#52)

Conflicts:
	autoload/openbrowser.vim
vitalize
fix #53: Vim loose a focus after opening URI
use FocusLost event instead (#53)
add popup-menus
add lower priority to openbrowser menus