Read fields/methods.csv for MCP directory mapping loading

You can now specify an MCP config directory with --srg-in and
the .srg will be loaded along with the fields.csv and methods.csv
mappings, providing descriptive "csv" names instead of the
numeric "srg" names. Example usage:

java -jar target/SpecialSource-1.3-SNAPSHOT-shaded.jar --in-jar ../jars/minecraft_server-147.jar --srg-in ../MinecraftForge/mcp/conf/ --out-jar /tmp/minecraft-server-pkgmcp.jar

For compiling mods against the minecraft-server-pkgmcp.jar
as a library, an alternative to recompiling all of Minecraft
by using MCP (much faster, like compiling Bukkit plugins).

Note: if the fields.csv and methods.csv are not present, the
numeric "srg" names will be loaded instead.