Add "Cash" portfolio option

[ConradSelig]

I don't always have every $ invested in one stock or another - but it would be nice to have the money by broker is holding for me show up in that "Total Value" line in the portfolio summary.

The "Cash" option should go in portfolio.ini.

...
[Cash]
amount=[ float ]
...

Things to consider:

• The ticker "Cash" does not exist - so this section should not be put though yfinanace.
• Cash will never need a graph.
• Cash does not need to show up in the portfolio table.
• An extra line in the summary should be added: "Cash Total: " (likely just below "Total Value:")