Entering metadata to transactions

I would like to enter some metadata to entries I'm logging, this could be done
as if it is another account taking part in the transaction without an amount
accompanying it. As here:
```
account0, 10.00
account1, -10.00
; :TAG:,
```
where each line corresponds to a line on the transaction entry view of the app.
Without a metadata line the same transaction could have been logged as:
```
account0, 10.00
account1,
```
This is expected and compatible with ledger syntax, since the second amount
would be assumed as `-10`. It would be more practical to parse the `;` there so
that it is excepted from the balance requirement and the entry could have been
done as here:
```
account0, 10.00
account1,
; :TAG:,
```
Which is a bit more practical and would look leaner on the ledger file.
Currently an attempt to create such a transaction has the save button grayed
out.

Please let me know if it is already possible in another way, which I couldn't
figure out.

Note: I couldn't reach to the webpage or find where the IRC channel would be so
the question ended up here.

version: 0.2.20 on Android

Start at the transaction entry view and trace the validation behind the disabled save button. Check how ledger transaction lines and semicolon-prefixed metadata are parsed, then verify that a transaction containing metadata can be saved without violating balance validation.

Written by the indexing model from the issue text.