Add a "completion trigger" option to completion files

For large completion files that take over the suggestions box, having a a "completion trigger" would be helpful to trigger certain completions, for example:

{

...

"completion trigger": "M-",

"completions":

[

...

]

}

would cause sublime to only suggest the completions in the file after the user types "M-". This could be used for auto completing common methods, like ".length()" in java would be completed with "M-length", but not when trying to type a variable name, like "length".