Add init file support

Create a file `javarepl.init` in the current folder and each line will be executed when the REPL is started up.

For example:
_javarepl.init_
```
final String foo = "foo";
```

Then the following will be reported by `:list` right after stating the REPL:
```
java> :list
Results:
    java.lang.String foo = "foo"

Types:


Imports:
    java.lang.*
    java.util.*
    java.math.*
    java.lang.Math.*
    java.util.function.*

Methods:
```