tab character is also ignored as same as whitespace
get rid of cosmetic characters from a:q_args
support appending output to buffer

* ':Capture' appends output to a capture buffer if capture window exists
* ':Capture!' opens a capture buffer each time (old behavior)
* add ':setfiletype capture'

Don't criticize me if you see this hunk.
I know this commit is ridiculous, I know...
avoid setting b:capture_bufnamenr and bufname at callee
:return at :catch should be callee
detect nested :Capture to avoid that :redir fails
cosmetic fix
autoload-ize
:redir END should be at :finally
error handling for 'execute <q-args>'
oops... :put just doesn't ignore a comment string...
add description about :Capture!
add description & code: current capture.vim window has higher priority