Support file mode

Current version of libbun2 does not support file mode.
This pull request supports that a user can load script both shell mode and file mode.
```
$ cat add.bun
1+2
$ java -jar libbun2.jar --peg sample/arithmetic.peg add.bun
...
Parsed: #add
#add {            ## by peg : null
   #integer: 1   ## by peg : null
   #integer: 2   ## by peg : null
}
```