Add g:snipmate_default_choice option

[javier-lopez]

When several snippets match a trigger in snipmate an input dialog is displayed asking for an option, the default behavour is to force the user to input a number and cancel the operation if none is selected, I prefer to press <Enter> to select the 1st option and only input a number when I decide to use another snippet or press <Esc> to cancel the operation.

This pull request adds a g:snipmate_default_option variable who accepts numerical values to pre-select an option on those cases, eg.

let g:snipmage_default_option = 1