Add ability to write math expression in amount field

[parthokunda]

When creating new expenses, currently there is no support to write mathematical expressions. My solution includes a package math-expression-evaluator. On change in the amount field, it tries to evaluate the string and shows the evaluated value if correct and 'Invalid expression' otherwise. The evaluated amount is shown below the input box. 'This is a reimbursement' is renamed to 'Reimbursement' and shifted to the right side of the total value field.

Fixes #183