Support installing specific versions of Miniconda

[jwodder]

Closes #157.

Replaces #156.

Fixes #155.

[codecov-commenter]

Codecov Report

Patch coverage: 83.33% and project coverage change: +62.78 :tada:

Comparison is base (c98f646) 0.00% compared to head (2f90ad7) 62.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #158 +/- ## =========================================== + Coverage 0 62.78% +62.78% =========================================== Files 0 1 +1 Lines 0 1470 +1470 Branches 0 290 +290 =========================================== + Hits 0 923 +923 - Misses 0 449 +449 - Partials 0 98 +98 ``` | [Impacted Files](https://codecov.io/gh/datalad/datalad-installer/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [src/datalad\_installer.py](https://codecov.io/gh/datalad/datalad-installer/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-c3JjL2RhdGFsYWRfaW5zdGFsbGVyLnB5) | `62.78% <83.33%> (ø)` | |

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[yarikoptic]

Looks great to me and as it is needed to be adopted by datalad -- I am marking it for the release. Thank you @jwodder !