Modifiers for amount required at retirement - External Contributions

[JackMorrissey]

This is a newly created issue from #4 because I wanted to close that issue for the dopamine hit.

Create an advanced Settings section that allows you to change the required amount at FI. This can be used to lower the amount needed to account for estimated pensions/social security. Additionally, we could go the other way and say you need "FI + 200k to pay off the mortgage if I wanted to" à la 1500 days

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From #4,

Something along the lines of:

With annual expenses of 40,148.52, we're aiming for a Net Worth of 1,003,713.

changing to

With annual expenses of 40,148.52, and expected external annual contributions of 12,000, we're aiming for a Net Worth of 703,713.