Add heading level to markdown option

[breyed]

Feature suggestion: For the --markdown option, add a heading level after it. Use 2 (the existing heading level) as the default.

This will allow projects that include an envinfo report in their issues to integrate the formatting with the rest of the issue formatting. For example, the NextAuth.js bug template, which places the report in a section with heading level 3, would direct contributors to run envinfo with heading level 4:

npx envinfo --system --binaries --browsers --npmPackages "next,next-auth,react" --markdown 4

The markdown would emit a number of "#"s according to the heading level. In the NextAuth.js example:

#### System:
 - OS: macOS 12.2
 - CPU: (8) x64 Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
 - Memory: 511.94 MB / 16.00 GB
 - Shell: 5.8 - /bin/zsh
#### Binaries:
 - Node: 16.13.2 - /usr/local/bin/node
 - Yarn: 1.22.15 - /usr/local/bin/yarn
 - npm: 8.4.1 - /usr/local/bin/npm
#### Browsers:
 - Chrome: 98.0.4758.80
 - Safari: 15.3
#### npmPackages:
 - next: ^12.0.10 => 12.0.10 
 - next-auth: ^4.2.1 => 4.2.1 
 - react: ^17.0.2 => 17.0.2