tiny C support; cosmetic improvements for the docs

doc/advopt.txt

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+Advanced commands:
+  //run                     run the project (with Tiny C backend; Linux only!)
+  //pretty                  pretty print the inputfile
+  //genDepend               generate a DOT file containing the
+                            module dependency graph
+  //listDef                 list all defined conditionals and exit
+  //check                   checks the project for syntax and semantic
+  //parse                   parses a single file (for debugging Nimrod)
+
+Advanced options:
+  -w, --warnings:on|off     turn all warnings on|off
+  --warning[X]:on|off       turn specific warning X on|off
+  --hints:on|off            turn all hints on|off
+  --hint[X]:on|off          turn specific hint X on|off
+  --lib:PATH                set the path to the system.nim library
+  -c, --compileOnly         compile only; do not assemble or link
+  --noLinking               compile but do not link
+  --noMain                  do not generate a main procedure
+  --genScript               generate a compile script (in the 'nimcache'
+                            subdirectory named 'compile_$project$scriptext')
+  --os:SYMBOL               set the target operating system (cross-compilation)
+  --cpu:SYMBOL              set the target processor (cross-compilation)
+  --debuginfo               enables debug information
+  -t, --passc:OPTION        pass an option to the C compiler
+  -l, --passl:OPTION        pass an option to the linker
+  --genMapping              generate a mapping file containing
+                            (Nimrod, mangled) identifier pairs
+  --lineDir:on|off          generation of #line directive on|off
+  --checkpoints:on|off      turn checkpoints on|off; for debugging Nimrod
+  --skipCfg                 do not read the general configuration file
+  --skipProjCfg             do not read the project's configuration file
+  --gc:refc|boehm|none      use Nimrod's native GC|Boehm GC|no GC
+  --index:FILE              use FILE to generate a documenation index file
+  --putenv:key=value        set an environment variable
+  --listCmd                 list the commands used to execute external programs
+  --parallelBuild=0|1|...   perform a parallel build
+                            value = number of processors (0 for auto-detect)
+  --verbosity:0|1|2|3       set Nimrod's verbosity level (0 is default)
+  -v, --version             show detailed version information