This is a fork from [select2][https://github.com/select2/select2], just extend a function, the select all. Here is [GITHUB][https://github.com/select2/select2] Here is [documentation][https://select2.org/]

• Change tag, delete string selectall

• Fix not working when multiple select have a same ID.

• Add select all for multiple select

• Add option selectAllOption: true, // select all button by default. This option is true by default, if it's multiple, then will display a checkbox select all, if set up false, then disable that.
• Add option selectAllText: 'Select all', // Select all text This option is Select all by default, that is the checkbox will be show what.