Allow inline currency conversion input

Cone version used: `Version 0.2.20`

Hi there,

Is there a chance to put inline currency conversion into the list of future features?

Here is one example of what i meant by that

```
2020-05-04 * Conversion
Pocket EUR 10,00 @ USD 1.10
Bank
```

This can quite be good since it works in ledger-cli and it is super convenient to convert and write down rates right away instead of defining changes right before you move your property between accounts.

So far, cone is allowing only numbers in that input field.

The issue does not name a file, test, or entry point. Start by locating the input parser and transaction-handling code, then verify that the example's inline conversion syntax is accepted and represented correctly in the application.

Written by the indexing model from the issue text.