Extract duplicated dotnet initialization into helper function

[mircealungu]

From review of PR #81.

The dotnet initialization block is copy-pasted identically in 4 places (render, render_json, render_diff, render_diff_json) in src/python/src/cli_interface.py:

from pythonnet import load
load("coreclr")
import clr
clr.AddReference(os.path.join(os.path.dirname(__file__), '.dotnet', 'Microsoft.CodeAnalysis.dll'))
clr.AddReference(os.path.join(os.path.dirname(__file__), '.dotnet', 'Archlens.dll'))
from Archlens.CLI import Program

Should be extracted into a helper like _init_dotnet() that returns Program.

Similarly, the plantuml rendering block (calling os.system with python -m plantuml) is duplicated in render and render_diff.