Implement a simple, robust and predictable KV parser

[dominikschulz]

This commit replaces the YAML parser with a very simple key value parser that doesn't suffer from YAML's weaknesses.

Fixes #559
Fixes #638

[codecov]

Codecov Report

Merging #659 into master will increase coverage by 0.01%.
The diff coverage is 75.42%.

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
+ Coverage    64.5%   64.52%   +0.01%     
==========================================
  Files         142      143       +1     
  Lines        7950     7967      +17     
==========================================
+ Hits         5128     5140      +12     
- Misses       2202     2204       +2     
- Partials      620      623       +3

Impacted Files	Coverage Δ
commands.go	83.47% <ø> (-0.28%)	⬇️
store/secret/secret.go	94.2% <72.73%> (+3.6%)	⬆️
store/secret/kv.go	75% <75%> (ø)
store/secret/yaml.go	76.6% <76.6%> (ø)

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[dominikschulz]

@notandy I used your code from #638 as a starting point and implemented a new K/V parser with YAML fallback support.

@martinhoefling This should cover #559 as well.

[notandy]

@dominikschulz works well with gopass-bridge, thanks!