Exports should show expressions and substitutions

[hrj]

Given a transaction like this:

. 2016/Sep/9     account1     xyz * pqr(3)

The exported post entry can include the expression verbatim, as well as the substituted values.

For journal export in XML format, this would be something like this:

<txn>
  <post delta="120" name="account1" expression="xyz*pqr(3)" substitution="10 * 12" />
  <post delta="-120" name="balancedAccount" />
</txn>

For ledger format, we could emit the expressions and substitutions as comments.

Alternatively, we could also achieve the same goal by,

• A builtin-function that returns the post's expression as a string
• A builtin-function that returns the post's substitutions as a string
• Ability to evaluate expressions in a comment!