[Datumaro] Convert command

[zhiltsov-max]

Motivation and context

Added a shortcut convert command to convert datasets. This is supposed to simplify tool usage.

How has this been tested?

Manual test

How to test:

datum convert -h
datum convert -i src/path -f dst_format [-if src_format]

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6373

• 10 of 72 (13.89%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 64.989%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/cli/commands/convert.py	10	72	13.89%

Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/components/extractor.py	1	91.01%

Totals
Change from base Build 6362:	-0.2%
Covered Lines:	11072
Relevant Lines:	16628

---

💛 - Coveralls

[nmanovic]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- datumaro/datumaro/cli/commands/convert.py  16
         

See the complete overview on Codacy

[nmanovic]

LGTM