Add option to center all text, and display encryption time by default

[bsdayo]

I learned Rust from scratch and implemented the features I mentioned in #88. 😄

There's a breaking change I made, to replace the passphrase field (Passphrase: _____) with encryption time (Encrypted at XXXX) by default. The passphrase field can be restored with -P option.

The reason I made this decision is that I think the vast majority of people don't write their passphrase directly on paper, resulting in wasted space being taken up. If you have any comments about this, please communicate with me.

Options added:

• -c, --center - Center all text
• -P, --passphrase-field - Display passphrase field instead of time

Closes #88

[bsdayo]

As a preview:

[codecov[bot]]

Codecov Report

Patch coverage: 60.65% and project coverage change: -3.28% :warning:

Comparison is base (ab53268) 95.71% compared to head (8c728c3) 92.43%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #89 +/- ## ========================================== - Coverage 95.71% 92.43% -3.28% ========================================== Files 7 7 Lines 677 701 +24 ========================================== Hits 648 648 - Misses 29 53 +24 ``` | [Files Changed](https://app.codecov.io/gh/matiaskorhonen/paper-age/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matias+Korhonen) | Coverage Δ | | |---|---|---| | [src/cli.rs](https://app.codecov.io/gh/matiaskorhonen/paper-age/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matias+Korhonen#diff-c3JjL2NsaS5ycw==) | `86.20% <50.00%> (-2.69%)` | :arrow_down: | | [src/builder.rs](https://app.codecov.io/gh/matiaskorhonen/paper-age/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matias+Korhonen#diff-c3JjL2J1aWxkZXIucnM=) | `88.38% <58.49%> (-7.97%)` | :arrow_down: | | [src/main.rs](https://app.codecov.io/gh/matiaskorhonen/paper-age/pull/89?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matias+Korhonen#diff-c3JjL21haW4ucnM=) | `87.85% <100.00%> (ø)` | |

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