www/nsm: Update to 1.20

Changelog picked from https://github.com/nifty-site-manager/nsm/releases:

Nift (aka nsm) v1.20
Release Notes:
    made template language available with input parameters
    added paginfo syntax @pagename, @pagepath, @contentpath, @templatepath to template language
    added siteinfo syntax @contentdir, @sitedir, @contentext, @pageext, @defaulttemplate to template language
    fixed indenting bugs
    fixed os_mtx functionality
    added optional sleepTime parameter for Nift serve command
Nift (aka nsm) v1.19
Release Notes:
    added more error handling
    added string variables
    added rootBranch and siteBranch to config files
    changed/improved/finalised how pre/post build/serve scripts are done
    fixed @script[output] and @System[output/content]
    added pre/post build-[all/updated] script support
Nift (aka nsm) v1.18
Release Notes:
    added FileSystem.[h/cpp] to the project
    added cpDir function to FileSystem.[h/cpp]
    renamed trash to ret_val and handle more errors
    fixed numerous minor bugs
Nift (aka nsm) v1.17
Release Notes:
    changed std::endl to "\n" when writing to file, 20% improvement in build-all time on some machines, no improvement on others
    changed pages set to pointer in PageBuilder.h, significantly less memory consumption
    added/improved Nift commands new-template, new-site-dir, new-cont-dir, new-cont-ext, new-page-ext
    added @systemcontent(sys-call) syntax to template language
    fixed bug with @System, @systemoutput, @script, @scriptoutput syntax
Nift (aka nsm) v1.16
Release Notes:
    improved multithreading
    added @inputhead syntax to the template language
    fixed read_sys_call and read_path
    improved new-page-ext
Nift (aka nsm) v1.15
Release Notes:
    added non-default page extension support
    added @dep syntax to the template language

(ng0)