Improve the Basilisp bootstrapping experience (#620)

* Improve the Basilisp bootstrapping experience

* Changelog

Author: chrisrink10

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+ * Added a bootstrapping function for easily bootstrapping Basilisp projects from Python (#620)
 
 ## [v0.1.dev15]
 ### Added

src/basilisp/main.py

@@ -1,19 +1,54 @@
 import importlib
 import logging
+from typing import Optional
 
 from basilisp import importer as importer
 from basilisp.lang import runtime as runtime
+from basilisp.lang.compiler import compiler_opts
 from basilisp.lang.typing import CompilerOpts
+from basilisp.lang.util import munge
 from basilisp.logconfig import DEFAULT_HANDLER, DEFAULT_LEVEL
 
 logger = logging.getLogger("basilisp")
 logger.setLevel(DEFAULT_LEVEL)
 logger.addHandler(DEFAULT_HANDLER)
 
 
-def init(compiler_opts: CompilerOpts) -> None:
-    """Initialize the runtime environment for evaluation."""
+def init(opts: Optional[CompilerOpts] = None) -> None:
+    """
+    Initialize the runtime environment for Basilisp code evaluation.
+
+    If you want to execute a Basilisp file which is stored in a well-formed package
+    or module structure, you probably want to use `bootstrap()` below.
+    """
     runtime.init_ns_var()
-    runtime.bootstrap_core(compiler_opts)
+    runtime.bootstrap_core(opts if opts is not None else compiler_opts())
     importer.hook_imports()
     importlib.import_module("basilisp.core")
+
+
+def bootstrap(
+    target: str, opts: Optional[CompilerOpts] = None
+) -> None:  # pragma: no cover
+    """
+    Import a Basilisp namespace or function identified by `target`.
+
+    Basilisp only needs to be bootstrapped once per Python VM invocation. Subsequent
+    imports of Basilisp namespaces will work using Python's standard `import` statement
+    and `importlib.import_module` function.
+
+    `target` must be a string naming a Basilisp namespace. Namespace references may
+    be given exactly as they are found in Basilisp code. `target` may optionally
+    include a trailing function reference, delimited by ":", which will be executed
+    after the target namespace is imported. If a function reference is given, the
+    function will be called with no arguments.
+
+    `opts` is a mapping of compiler options that may be supplied for bootstrapping.
+    This setting should be left alone unless you know what you are doing.
+    """
+    init(opts=opts)
+    pkg_name, *rest = target.split(":", maxsplit=1)
+    mod = importlib.import_module(munge(pkg_name))
+    if rest:
+        fn_name = munge(rest[0])
+        getattr(mod, fn_name)()