tex2svg fails if input is a number or starts with "-"

[Wallacoloo]

For example:

$ tex2svg '2'
/usr/lib/node_modules/tex-equation-to-svg/lib/factory.js:51
			throw new TypeError( 'invalid input argument. Must provide a string primitive. Value: `' + str + '`.' );

TypeError: invalid input argument. Must provide a string primitive. Value: `undefined`.
    at tex2svg (/usr/lib/node_modules/tex-equation-to-svg/lib/factory.js:51:10)
    at tex2svg (/usr/lib/node_modules/tex-equation-to-svg/lib/tex2svg.js:28:26)
    at Object.<anonymous> (/usr/lib/node_modules/tex-equation-to-svg/bin/cli:85:1)
    at Module._compile (module.js:571:32)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)
    at Module.runMain (module.js:605:10)
    at run (bootstrap_node.js:427:7)

On the other hand,

$ tex2svg ' 2'

(notice the extra space before the minus sign) and

$ tex2svg '2 < a'

both produce valid outputs.

Note that tex2svg '-2 < a' still breaks. "-" seems to be deadly when occurring as the first character.