Add load/save methods to Board

[sigfrido]

Currently load and save methods use json and belong to the Console class

• Add load/save methods to Board class
• Use a plain string format
• Board size should be determined by the width of the strings

In reading, we could support different formats.

Moves

<row>[sep]<col>[sep]<value>
...

Board

<col1>..<coln>
... exactly n lines