fix issue of parse_args not handling ':' in options correctly

[fotovlieger]

If an option contains ':', the parsed value is truncated
test:

--package.path  = '/home/hel/Penlight/lua/?.lua' .. ';' .. package.path
app=require 'pl.app'
pretty=require 'pl.pretty'
my_args = {'hello', '--flip=this', '--flop=that:zzz'}
args = app.parse_args(my_args)
print(pretty.write(args))

prints:

{
  flop = "that",
  flip = "this"
}

flop should be 'that:zzz'
Note: this is my first pull request ever, sure hope it works out :-)

[Tieske]

thx @fotovlieger . I implemented it slightly differently in #373 by extending the splitv app to do "the right thing", and added some tests.

Thanks for your contribution!

[Tieske]

replaced by #373