Add postfix arg to MetricCollection

[SkafteNicki]

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What does this PR do?

Fixes #168
Adds a postfix arg that behaves similar to the prefix arg in metric collection.

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[codecov]

Codecov Report

Merging #188 (2ecd19d) into master (c1ae5ac) will increase coverage by 0.02%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   96.05%   96.08%   +0.02%     
==========================================
  Files         180       90      -90     
  Lines        5760     2886    -2874     
==========================================
- Hits         5533     2773    -2760     
+ Misses        227      113     -114     

Flag	Coverage Δ
Linux	79.31% <66.66%> (-0.03%)	⬇️
Windows	79.31% <66.66%> (-0.03%)	⬇️
cpu	96.08% <94.44%> (+<0.01%)	⬆️
gpu	?
macOS	96.08% <94.44%> (+<0.01%)	⬆️
pytest	96.08% <94.44%> (+0.02%)	⬆️
python3.6	96.00% <94.44%> (+<0.01%)	⬆️
python3.8	96.01% <94.44%> (+<0.01%)	⬆️
python3.9	95.91% <94.44%> (+<0.01%)	⬆️
torch1.3.1	94.85% <94.44%> (+0.01%)	⬆️
torch1.4.0	94.97% <94.44%> (+0.01%)	⬆️
torch1.8.1	95.91% <94.44%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
torchmetrics/collections.py	96.96% <94.44%> (+0.30%)	⬆️
...chmetrics/functional/classification/stat_scores.py
.../s/torchmetrics/classification/confusion_matrix.py
__w/2/s/torchmetrics/classification/f_beta.py
__w/2/s/torchmetrics/functional/regression/psnr.py
__w/2/s/torchmetrics/classification/auc.py
__w/2/s/torchmetrics/setup_tools.py
__w/2/s/torchmetrics/classification/stat_scores.py
...torchmetrics/functional/classification/__init__.py
__w/2/s/torchmetrics/average.py
... and 81 more

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