Add Matrix Market Exchange Formats support

## Language name

[Matrix Market Exchange Formats](https://math.nist.gov/MatrixMarket/formats.html#MMformat)

## URL of example repository

https://github.com/jip/mt

## URL of syntax highlighting grammar

(none)

The format is very simple. It is human readable and ASCII-based; items are case insensitive and blank-delimited; extra blanks between items are ignored. A file begins with header lines, then data lines follow. All lines are limited to 1024 characters. The first header line begins with signature text item `%%MatrixMarket `, followed by some items in the same line. The rest header lines must begin with `%`, they are comments and may be absent. The data lines contain numbers only delimited by blanks; leading and trailing blanks are ignored. Numbers are integers and floats in decimal and exponential notation common to C and Fortran. Blank lines may appear anywhere after the first line.

## Most popular extensions

`.mtx` in Matlab, Octave, Python

## Detected language

[Objective-C++](https://github.com/jip/mt/search?l=objective-c%2B%2B)