[Breaking] Add support for watching multiple paths

[florimondmanca]

Fixes #11

Migration steps:

hotreload = arel.HotReload("./path", on_reload=[...])

becomes:

hotreload = arel.HotReload(
    paths=[
        arel.Path("./path", on_reload=[...]),
    ],
)

[codecov-commenter]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9        11    +2     
  Lines          174       189   +15     
=========================================
+ Hits           174       189   +15     

Impacted Files	Coverage Δ
src/arel/__init__.py	100.00% <100.00%> (ø)
src/arel/_app.py	100.00% <100.00%> (ø)
src/arel/_models.py	100.00% <100.00%> (ø)
src/arel/_types.py	100.00% <100.00%> (ø)
tests/test_example.py	100.00% <100.00%> (ø)

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