allow modification of version via hooks

The new internal version-munge command is used to modify the version when
read from a version file or when specified on the command line via the
shell, local, or global commands.  The version stored in a version
file is not modified.

This allows the user to modify the version dependent upon the existing
environment.  For example, if the PLENV_ROOT directory is shared
across multiple platforms with different OS's, hardware, or ABI's, one
could modify the version at run-time to include a platform specific
suffix, allowing a single version stored in a version file to
accomodate multiple platforms.

For example, if the hostname is used to distinguish the platform, e.g.
on each host Perl was installed with

    % plenv install --as 5.22-$(hostname) 5.22

Then with the following hook:

    % cat ~/.plenv/plenv.d/version-munge/hostname.bash
    #!/bin/bash

    if [[ "$version" != --* ]]; then

	suffix=$(hostname)

	plenv-prefix "$version-$suffix" 1> /dev/null \
	    && version="$version-$suffix"

    fi

Setting

    % plenv global 5.22

will set ~/.plenv/version to 5.22 and the hook will ensure that
each host will see its own version.

libexec/plenv-sh-shell

@@ -27,6 +27,8 @@ version="$1"
 shell="${PLENV_SHELL:-$SHELL}"
 shell="${shell##*/}"
 
+version=$(plenv-version-munge $version)
+
 if [ -z "$version" ]; then
   if [ -z "$PLENV_VERSION" ]; then
     echo "plenv: no shell-specific version configured" >&2

libexec/plenv-version-file-read

@@ -3,5 +3,9 @@
 # Reads the first non-whitespace word from the specified version file,
 # careful not to load it whole in case there's something crazy in it.
 if [ -e "$1" ]; then
-  command -p awk 'BEGIN {rc=1} NF > 0 { print $1 ; exit rc=0 } END { exit rc }' "$1"
+
+  version=$(command -p awk 'BEGIN {rc=1} NF > 0 { print $1 ; exit rc=0 } END { exit rc }' "$1")
+
+  plenv-version-munge "$version"
+
 fi

libexec/plenv-version-file-write

@@ -13,7 +13,7 @@ if [ -z "$PLENV_VERSION" ] || [ -z "$PLENV_VERSION_FILE" ]; then
 fi
 
 # Make sure the specified version is installed.
-plenv-prefix "$PLENV_VERSION" >/dev/null
+plenv-prefix "$(plenv-version-munge \"$PLENV_VERSION\")" >/dev/null
 
 # Write the version out to disk.
 echo "$PLENV_VERSION" > "$PLENV_VERSION_FILE"

libexec/plenv-version-munge

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Summary: munge the passed Perl version using plugins
+
+version="$1"
+if [ "$version" != '' ]; then
+
+    OLDIFS="$IFS"
+    IFS=$'\n' scripts=(`plenv-hooks version-munge`)
+    IFS="$OLDIFS"
+
+    for script in "${scripts[@]}"; do
+	source "$script"
+    done
+
+fi
+
+echo "$version"