Add unchecked TOTP constructor and URL parsing methods

[timvisee]

Fixes #46

This adds TOTP::new_unchecked as discussed.

This also adds TOTP::from_url_unchecked which uses TOTP::new_unchecked internally to allow unchecked URL parsing.

I will rebase #47 after this is merged.

[timvisee]

I just noticed I didn't add tests for this. I'll do that later.

[codecov-commenter]

Codecov Report

Base: 96.69% // Head: 95.81% // Decreases project coverage by -0.88% ⚠️

Coverage data is based on head (d293068) compared to base (d460bd2).
Patch coverage: 75.47% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   96.69%   95.81%   -0.89%     
==========================================
  Files           4        4              
  Lines        1242     1290      +48     
==========================================
+ Hits         1201     1236      +35     
- Misses         41       54      +13     

Impacted Files	Coverage Δ
src/lib.rs	94.65% <75.47%> (-1.39%)	⬇️

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[constantoine]

LGTM!