Improve code coverage support

Improve code coverage support. Now will write out coverage reports in ht...ml, lcov, cobertura, clover, and json to ./coverage.

Run venus in coverage mode:

```bash
venus folder_with_tests -c -n
```

Get coverage summary at end:

```bash
=============================== Coverage summary ===============================
Statements  : 67.21% ( 41/61 )
Branches     : 16.67% ( 2/12 )
Functions    : 80% ( 16/20 )
Lines           : 67.21% ( 41/61 )
================================================================================
```

Coverage reports written to `./coverage` (relative to folder where `venus` was executed):

```bash
coverage
├── clover.xml
├── cobertura-coverage.xml
├── coverage-final.json
├── index.html
├── lcov-report
│   ├── 1
│   ├── 2
│   ├── 3
│   ├── 4
│   ├── 6
│   ├── 7
│   ├── index.html
│   ├── prettify.css
│   └── prettify.js
├── lcov.info
├── prettify.css
```