Speeding up Blurry with Python's concurrent.futures

A clean build of https://johnfraney.ca, which is built using Blurry.

Before:

❯ time blurry build
.  .             
|-.| . ..-..-.. .
`-''-'-''  '  '-|
              `-'
┏━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Markdown Plugins    ┃ HTML Plugins ┃ Jinja Plugins ┃
┡━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ container           │ minify_html  │ blurry_image  │
│ punctuation         │              │               │
│ python_code         │              │               │
│ python_code_in_list │              │               │
└─────────────────────┴──────────────┴───────────────┘
Blurring 36 Markdown files from: content
Built site in 56.14843 seconds

________________________________________________________
Executed in   57.45 secs    fish           external
   usr time   69.62 secs  210.00 micros   69.62 secs
   sys time    0.97 secs   74.00 micros    0.97 secs

After:

❯ time blurry build
.  .             
|-.| . ..-..-.. .
`-''-'-''  '  '-|
              `-'
┏━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Markdown Plugins    ┃ HTML Plugins ┃ Jinja Plugins ┃
┡━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ container           │ minify_html  │ blurry_image  │
│ punctuation         │              │               │
│ python_code         │              │               │
│ python_code_in_list │              │               │
└─────────────────────┴──────────────┴───────────────┘
Blurring 37 Markdown files and 98 other files from: content
Built site in 27.084359 seconds

________________________________________________________
Executed in   28.50 secs    fish           external
   usr time  108.06 secs  258.00 micros  108.06 secs
   sys time    3.14 secs    0.00 micros    3.14 secs