refactor: mutiple key support

[ShuguangSun]

Add support for multiple key for one snippet. For example, the snippet below will be parsed as two snippets with two keys.

# name: choose
# key: choose
# key: choosevalue
# --
\\\"\${${1:1}:$$(yas-choose-value '("$0"))}\\\"

To avoid introducing bugs, I make a copy of some functions, for example, yas--parse-template to yas--parse-template-for-mulkey, and make changes to those functions.
M-x yas-compile-directory-for-mulkey for JIT.

As this patch intends to modifide the funtion yas--parse-template which is a quite core function, I'm not sure whether this patch can be accepted. If it is not acceptable now, I'd like to ask your kindness agreement for me to make a separate package, like the one in
yasnippet-multiple-key. It uses your code and the patch.

[ShuguangSun]

Related with:
#375
#114
#342