Feature request: sqlite-utils insert-files should be able to convert fields

[grimnight]

Currently using both insert-files and convert is needed in order to create sqlar files, it would be more convenient if it could be done with just one command.

~
❯ cat test.py
import os

class Example:
    def __init__(self, arg1, arg2):
        self.arg1 = arg1

~
❯ sqlite-utils insert-files test.sqlar sqlar test.py -c name:name -c data:content -c mode:mode -c mtime:mtime -c sz:size --pk=name
  [####################################]  100%

~
❯ sqlite-utils convert test.sqlar sqlar data "zlib.compress(value)" --import=zlib --where "name = 'test.py'"
[####################################]  100%

~
❯ cat test.py | sqlite-utils convert test.sqlar sqlar data "zlib.compress(sys.stdin.buffer.read())" --import=zlib --import=sys --where "name = 'test.py'" # Alternative way
  [####################################]  100%

~
❯ sqlite3 test.sqlar "SELECT hex(data) FROM sqlar WHERE name = 'test.py';" | python3 -c "import sys, zlib; sys.stdout.buffer.write(zlib.decompress(bytes.fromhex(sys.stdin.read())))"
import os

class Example:
    def __init__(self, arg1, arg2):
        self.arg1 = arg1

~
❯ rm test.py

~
❯ sqlar -l test.sqlar
test.py

~
❯ sqlar -x test.sqlar

~
❯ cat test.py
import os

class Example:
    def __init__(self, arg1, arg2):
        self.arg1 = arg1