Feature/detect casting comb token from converters

[jorenretel]

Hi,

first of all: thanks for this amazing library. This makes life a lot easier.

Main

I have a suggestion, but I can not see all the implications it has. What I suggest is to suggest to change the regex that detects whether two casting tokens are used in one variable definition. I think it would be nice if this regex is generated dynamically based on the values in the "converters" dict. What this allows is that a user could add a custom casting token. For instance, I wanted a casting token that would cast the string to a pathlib.Path object (see example/custom_cast_token):

# app.py
from pathlib import Path
from dynaconf.utils import parse_conf

parse_conf.converters["@path"] = (
    lambda value: value.set_casting(Path)
    if isinstance(value, parse_conf.Lazy)
    else Path(value)
)

#settings.toml
some_path = "@path /home/john/foo.txt"

This last thing would also work without changing the regex. But it becomes a problem when I would like to do something like:

# settings.toml

parent = "@path @format {env[HOME]}/parent"
child = "@path @format {this.parent}/child"

The regex as it is now does not pick up "@path @Format" and therefore does not do the format step. This PR would solve that.

As as note. I also, found another solution that does not require changing Dynaconf:

# app.py
from pathlib import Path
parse_conf.converters["@path"] = lambda value: parse_conf.Lazy(value).set_casting(Path)

with the following:

# settings.toml
parent = "@path {env[HOME]}/parent"
child = "@path {this.parent}/child"

but this feels slightly hacky and would not allow to have casting to path without also formatting (which the Lazy does).

Tests

Before submitting this pull request I ran "make test_only" and that all passes. I also ran "make all", there I ran into some "maximum recursion depth reached" error. I already had this problem before making any changes though. I followed https://github.com/dynaconf/dynaconf/blob/master/CONTRIBUTING.md , but I must be doing something wrong. I have a similar (but not exactly the same) Traceback when I don't initialize settings like this:

settings = Dynaconf(
    envvar_prefix="DYNACONF",
    settings_files=["settings.toml"],
    environments=True,
)

but import them like I see in the other examples:

from dynaconf import settings